Merge tag 'rcu.2022.01.09a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu

Pull RCU updates from Paul McKenney:

 - Documentation updates, perhaps most notably Neil Brown's writeup of
   the reference-counting analogy to RCU.

 - Expedited grace-period cleanups.

 - Remove CONFIG_RCU_FAST_NO_HZ due to lack of valid users. I have asked
   around, posted a blog entry, and sent this series to LKML without
   result.

 - Miscellaneous fixes.

 - RCU callback offloading updates, perhaps most notably Frederic
   Weisbecker's updates allowing CPUs booted in the de-offloaded state
   to be offloaded at runtime.

 - nolibc fixes from Willy Tarreau and Anmar Faizi, but also including
   Mark Brown's addition of gettid().

 - RCU Tasks Trace fixes, including changes that increase the
   scalability of call_rcu_tasks_trace() for the BPF folks (Martin Lau
   and KP Singh).

 - Various fixes including those from Wander Lairson Costa and Li
   Zhijian.

 - Fixes plus addition of tests for the increased call_rcu_tasks_trace()
   scalability.

* tag 'rcu.2022.01.09a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu: (87 commits)
  rcu/nocb: Merge rcu_spawn_cpu_nocb_kthread() and rcu_spawn_one_nocb_kthread()
  rcu/nocb: Allow empty "rcu_nocbs" kernel parameter
  rcu/nocb: Create kthreads on all CPUs if "rcu_nocbs=" or "nohz_full=" are passed
  rcu/nocb: Optimize kthreads and rdp initialization
  rcu/nocb: Prepare nocb_cb_wait() to start with a non-offloaded rdp
  rcu/nocb: Remove rcu_node structure from nocb list when de-offloaded
  rcu-tasks: Use fewer callbacks queues if callback flood ends
  rcu-tasks: Use separate ->percpu_dequeue_lim for callback dequeueing
  rcu-tasks: Use more callback queues if contention encountered
  rcu-tasks: Avoid raw-spinlocked wakeups from call_rcu_tasks_generic()
  rcu-tasks: Count trylocks to estimate call_rcu_tasks() contention
  rcu-tasks: Add rcupdate.rcu_task_enqueue_lim to set initial queueing
  rcu-tasks: Make rcu_barrier_tasks*() handle multiple callback queues
  rcu-tasks: Use workqueues for multiple rcu_tasks_invoke_cbs() invocations
  rcu-tasks: Abstract invocations of callbacks
  rcu-tasks: Abstract checking of callback lists
  rcu-tasks: Add a ->percpu_enqueue_lim to the rcu_tasks structure
  rcu-tasks: Inspect stalled task's trc state in locked state
  rcu-tasks: Use spin_lock_rcu_node() and friends
  rcutorture: Combine n_max_cbs from all kthreads in a callback flood
  ...

16 files changed, 205 insertions, 112 deletions

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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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          y="268.18076">WRITE_ONCE(a, 1);</tspan></text>
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        xml:space="preserve"
-       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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@@ -157,7 +157,7 @@
          y="439.13766">WRITE_ONCE(b, 1);</tspan></text>
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        xml:space="preserve"
-       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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        id="text4445"
@@ -168,7 +168,7 @@
          y="309.29346">r1 = READ_ONCE(a);</tspan></text>
     <text
        xml:space="preserve"
-       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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        id="text4449"
@@ -179,7 +179,7 @@
          y="520.61786">WRITE_ONCE(c, 1);</tspan></text>
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        xml:space="preserve"
-       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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        id="text4453"
@@ -190,7 +190,7 @@
          y="384.71124">r2 = READ_ONCE(b);</tspan></text>
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        xml:space="preserve"
-       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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        id="text4457"
@@ -201,7 +201,7 @@
          y="582.13617">r3 = READ_ONCE(c);</tspan></text>
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        xml:space="preserve"
-       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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        y="213.91006"
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@@ -212,7 +212,7 @@
          y="213.91006">thread0()</tspan></text>
     <text
        xml:space="preserve"
-       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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@@ -223,7 +223,7 @@
          y="213.91006">thread1()</tspan></text>
     <text
        xml:space="preserve"
-       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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@@ -251,7 +251,7 @@
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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        id="text4429-8"
@@ -262,7 +262,7 @@
          y="251.53981">rcu_read_lock();</tspan></text>
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        xml:space="preserve"
-       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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@@ -273,7 +273,7 @@
          y="367.91556">rcu_read_lock();</tspan></text>
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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@@ -284,7 +284,7 @@
          y="597.40289">rcu_read_unlock();</tspan></text>
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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@@ -324,7 +324,7 @@
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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@@ -361,7 +361,7 @@
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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diff --git a/Documentation/RCU/Design/Requirements/ReadersPartitionGP1.svg b/Documentation/RCU/Design/Requirements/ReadersPartitionGP1.svg
index 48cd1623d4d4..e2a8af592bab 100644
--- a/Documentation/RCU/Design/Requirements/ReadersPartitionGP1.svg
+++ b/Documentation/RCU/Design/Requirements/ReadersPartitionGP1.svg
@@ -116,7 +116,7 @@
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        id="flowRoot2985"
-       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"><flowRegion
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"><flowRegion
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+         style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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@@ -163,7 +163,7 @@
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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        xml:space="preserve"
-       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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@@ -185,7 +185,7 @@
          y="487.13766">WRITE_ONCE(b, 1);</tspan></text>
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        xml:space="preserve"
-       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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@@ -196,7 +196,7 @@
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-       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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@@ -207,7 +207,7 @@
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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@@ -229,7 +229,7 @@
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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@@ -240,7 +240,7 @@
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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@@ -251,7 +251,7 @@
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@@ -281,7 +281,7 @@
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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@@ -292,7 +292,7 @@
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-       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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@@ -303,7 +303,7 @@
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-       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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@@ -314,7 +314,7 @@
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+         style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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@@ -453,7 +453,7 @@
     </g>
     <text
        xml:space="preserve"
-       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
        x="541.70508"
        y="387.6217"
        id="text4445-0"
@@ -464,7 +464,7 @@
          y="387.6217">r3 = READ_ONCE(d);</tspan></text>
     <text
        xml:space="preserve"
-       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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        y="427.29443"
        id="text4648-4-3-1"
@@ -499,7 +499,7 @@
          y="427.29443">QS</tspan></text>
     <text
        xml:space="preserve"
-       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
        x="686.27747"
        y="461.83929"
        id="text4453-7"
@@ -510,7 +510,7 @@
          y="461.83929">r4 = READ_ONCE(b);</tspan></text>
     <text
        xml:space="preserve"
-       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
        x="686.27747"
        y="669.26422"
        id="text4457-9"
@@ -521,7 +521,7 @@
          y="669.26422">r5 = READ_ONCE(e);</tspan></text>
     <text
        xml:space="preserve"
-       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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        id="text4429-8-9-33"
@@ -532,7 +532,7 @@
          y="445.04358">rcu_read_lock();</tspan></text>
     <text
        xml:space="preserve"
-       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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        id="text4429-8-9-3-8"
@@ -543,7 +543,7 @@
          y="684.53094">rcu_read_unlock();</tspan></text>
     <text
        xml:space="preserve"
-       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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        id="text4461-2-5"
@@ -626,7 +626,7 @@
          y="213.91006">thread3()</tspan></text>
     <text
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"
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diff --git a/Documentation/RCU/stallwarn.rst b/Documentation/RCU/stallwarn.rst
index 28f8ad16db25..78404625bad2 100644
--- a/Documentation/RCU/stallwarn.rst
+++ b/Documentation/RCU/stallwarn.rst
@@ -254,17 +254,6 @@ period (in this case 2603), the grace-period sequence number (7075), and
 an estimate of the total number of RCU callbacks queued across all CPUs
 (625 in this case).
 
-In kernels with CONFIG_RCU_FAST_NO_HZ, more information is printed
-for each CPU::
-
-	0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 dyntick_enabled: 1
-
-The "last_accelerate:" prints the low-order 16 bits (in hex) of the
-jiffies counter when this CPU last invoked rcu_try_advance_all_cbs()
-from rcu_needs_cpu() or last invoked rcu_accelerate_cbs() from
-rcu_prepare_for_idle(). "dyntick_enabled: 1" indicates that dyntick-idle
-processing is enabled.
-
 If the grace period ends just as the stall warning starts printing,
 there will be a spurious stall-warning message, which will include
 the following::
diff --git a/Documentation/RCU/whatisRCU.rst b/Documentation/RCU/whatisRCU.rst
index 17e95ab2a201..c34d2212eaca 100644
--- a/Documentation/RCU/whatisRCU.rst
+++ b/Documentation/RCU/whatisRCU.rst
@@ -39,9 +39,11 @@ different paths, as follows:
 
 :ref:`6.	ANALOGY WITH READER-WRITER LOCKING <6_whatisRCU>`
 
-:ref:`7.	FULL LIST OF RCU APIs <7_whatisRCU>`
+:ref:`7.	ANALOGY WITH REFERENCE COUNTING <7_whatisRCU>`
 
-:ref:`8.	ANSWERS TO QUICK QUIZZES <8_whatisRCU>`
+:ref:`8.	FULL LIST OF RCU APIs <8_whatisRCU>`
+
+:ref:`9.	ANSWERS TO QUICK QUIZZES <9_whatisRCU>`
 
 People who prefer starting with a conceptual overview should focus on
 Section 1, though most readers will profit by reading this section at
@@ -677,7 +679,7 @@ Quick Quiz #1:
 		occur when using this algorithm in a real-world Linux
 		kernel?  How could this deadlock be avoided?
 
-:ref:`Answers to Quick Quiz <8_whatisRCU>`
+:ref:`Answers to Quick Quiz <9_whatisRCU>`
 
 5B.  "TOY" EXAMPLE #2: CLASSIC RCU
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -732,7 +734,7 @@ Quick Quiz #2:
 		Give an example where Classic RCU's read-side
 		overhead is **negative**.
 
-:ref:`Answers to Quick Quiz <8_whatisRCU>`
+:ref:`Answers to Quick Quiz <9_whatisRCU>`
 
 .. _quiz_3:
 
@@ -741,7 +743,7 @@ Quick Quiz #3:
 		critical section, what the heck do you do in
 		CONFIG_PREEMPT_RT, where normal spinlocks can block???
 
-:ref:`Answers to Quick Quiz <8_whatisRCU>`
+:ref:`Answers to Quick Quiz <9_whatisRCU>`
 
 .. _6_whatisRCU:
 
@@ -872,7 +874,79 @@ be used in place of synchronize_rcu().
 
 .. _7_whatisRCU:
 
-7.  FULL LIST OF RCU APIs
+7.  ANALOGY WITH REFERENCE COUNTING
+-----------------------------------
+
+The reader-writer analogy (illustrated by the previous section) is not
+always the best way to think about using RCU.  Another helpful analogy
+considers RCU an effective reference count on everything which is
+protected by RCU.
+
+A reference count typically does not prevent the referenced object's
+values from changing, but does prevent changes to type -- particularly the
+gross change of type that happens when that object's memory is freed and
+re-allocated for some other purpose.  Once a type-safe reference to the
+object is obtained, some other mechanism is needed to ensure consistent
+access to the data in the object.  This could involve taking a spinlock,
+but with RCU the typical approach is to perform reads with SMP-aware
+operations such as smp_load_acquire(), to perform updates with atomic
+read-modify-write operations, and to provide the necessary ordering.
+RCU provides a number of support functions that embed the required
+operations and ordering, such as the list_for_each_entry_rcu() macro
+used in the previous section.
+
+A more focused view of the reference counting behavior is that,
+between rcu_read_lock() and rcu_read_unlock(), any reference taken with
+rcu_dereference() on a pointer marked as ``__rcu`` can be treated as
+though a reference-count on that object has been temporarily increased.
+This prevents the object from changing type.  Exactly what this means
+will depend on normal expectations of objects of that type, but it
+typically includes that spinlocks can still be safely locked, normal
+reference counters can be safely manipulated, and ``__rcu`` pointers
+can be safely dereferenced.
+
+Some operations that one might expect to see on an object for
+which an RCU reference is held include:
+
+ - Copying out data that is guaranteed to be stable by the object's type.
+ - Using kref_get_unless_zero() or similar to get a longer-term
+   reference.  This may fail of course.
+ - Acquiring a spinlock in the object, and checking if the object still
+   is the expected object and if so, manipulating it freely.
+
+The understanding that RCU provides a reference that only prevents a
+change of type is particularly visible with objects allocated from a
+slab cache marked ``SLAB_TYPESAFE_BY_RCU``.  RCU operations may yield a
+reference to an object from such a cache that has been concurrently
+freed and the memory reallocated to a completely different object,
+though of the same type.  In this case RCU doesn't even protect the
+identity of the object from changing, only its type.  So the object
+found may not be the one expected, but it will be one where it is safe
+to take a reference or spinlock and then confirm that the identity
+matches the expectations.
+
+With traditional reference counting -- such as that implemented by the
+kref library in Linux -- there is typically code that runs when the last
+reference to an object is dropped.  With kref, this is the function
+passed to kref_put().  When RCU is being used, such finalization code
+must not be run until all ``__rcu`` pointers referencing the object have
+been updated, and then a grace period has passed.  Every remaining
+globally visible pointer to the object must be considered to be a
+potential counted reference, and the finalization code is typically run
+using call_rcu() only after all those pointers have been changed.
+
+To see how to choose between these two analogies -- of RCU as a
+reader-writer lock and RCU as a reference counting system -- it is useful
+to reflect on the scale of the thing being protected.  The reader-writer
+lock analogy looks at larger multi-part objects such as a linked list
+and shows how RCU can facilitate concurrency while elements are added
+to, and removed from, the list.  The reference-count analogy looks at
+the individual objects and looks at how they can be accessed safely
+within whatever whole they are a part of.
+
+.. _8_whatisRCU:
+
+8.  FULL LIST OF RCU APIs
 -------------------------
 
 The RCU APIs are documented in docbook-format header comments in the
@@ -1035,9 +1109,9 @@ g.	Otherwise, use RCU.
 Of course, this all assumes that you have determined that RCU is in fact
 the right tool for your job.
 
-.. _8_whatisRCU:
+.. _9_whatisRCU:
 
-8.  ANSWERS TO QUICK QUIZZES
+9.  ANSWERS TO QUICK QUIZZES
 ----------------------------
 
 Quick Quiz #1:
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 49f495c9a7f8..685645e75706 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4365,19 +4365,30 @@
 				Disable the Correctable Errors Collector,
 				see CONFIG_RAS_CEC help text.
 
-	rcu_nocbs=	[KNL]
-			The argument is a cpu list, as described above.
-
-			In kernels built with CONFIG_RCU_NOCB_CPU=y, set
-			the specified list of CPUs to be no-callback CPUs.
-			Invocation of these CPUs' RCU callbacks will be
-			offloaded to "rcuox/N" kthreads created for that
-			purpose, where "x" is "p" for RCU-preempt, and
-			"s" for RCU-sched, and "N" is the CPU number.
-			This reduces OS jitter on the offloaded CPUs,
-			which can be useful for HPC and real-time
-			workloads.  It can also improve energy efficiency
-			for asymmetric multiprocessors.
+	rcu_nocbs[=cpu-list]
+			[KNL] The optional argument is a cpu list,
+			as described above.
+
+			In kernels built with CONFIG_RCU_NOCB_CPU=y,
+			enable the no-callback CPU mode, which prevents
+			such CPUs' callbacks from being invoked in
+			softirq context.  Invocation of such CPUs' RCU
+			callbacks will instead be offloaded to "rcuox/N"
+			kthreads created for that purpose, where "x" is
+			"p" for RCU-preempt, "s" for RCU-sched, and "g"
+			for the kthreads that mediate grace periods; and
+			"N" is the CPU number. This reduces OS jitter on
+			the offloaded CPUs, which can be useful for HPC
+			and real-time workloads.  It can also improve
+			energy efficiency for asymmetric multiprocessors.
+
+			If a cpulist is passed as an argument, the specified
+			list of	CPUs is set to no-callback mode from boot.
+
+			Otherwise, if the '=' sign and the cpulist
+			arguments are omitted, no CPU will be set to
+			no-callback mode from boot but the mode may be
+			toggled at runtime via cpusets.
 
 	rcu_nocb_poll	[KNL]
 			Rather than requiring that offloaded CPUs
@@ -4511,10 +4522,6 @@
 			on rcutree.qhimark at boot time and to zero to
 			disable more aggressive help enlistment.
 
-	rcutree.rcu_idle_gp_delay= [KNL]
-			Set wakeup interval for idle CPUs that have
-			RCU callbacks (RCU_FAST_NO_HZ=y).
-
 	rcutree.rcu_kick_kthreads= [KNL]
 			Cause the grace-period kthread to get an extra
 			wake_up() if it sleeps three times longer than
@@ -4625,8 +4632,12 @@
 			in seconds.
 
 	rcutorture.fwd_progress= [KNL]
-			Enable RCU grace-period forward-progress testing
+			Specifies the number of kthreads to be used
+			for  RCU grace-period forward-progress testing
 			for the types of RCU supporting this notion.
+			Defaults to 1 kthread, values less than zero or
+			greater than the number of CPUs cause the number
+			of CPUs to be used.
 
 	rcutorture.fwd_progress_div= [KNL]
 			Specify the fraction of a CPU-stall-warning
@@ -4827,6 +4838,29 @@
 			period to instead use normal non-expedited
 			grace-period processing.
 
+	rcupdate.rcu_task_collapse_lim= [KNL]
+			Set the maximum number of callbacks present
+			at the beginning of a grace period that allows
+			the RCU Tasks flavors to collapse back to using
+			a single callback queue.  This switching only
+			occurs when rcupdate.rcu_task_enqueue_lim is
+			set to the default value of -1.
+
+	rcupdate.rcu_task_contend_lim= [KNL]
+			Set the minimum number of callback-queuing-time
+			lock-contention events per jiffy required to
+			cause the RCU Tasks flavors to switch to per-CPU
+			callback queuing.  This switching only occurs
+			when rcupdate.rcu_task_enqueue_lim is set to
+			the default value of -1.
+
+	rcupdate.rcu_task_enqueue_lim= [KNL]
+			Set the number of callback queues to use for the
+			RCU Tasks family of RCU flavors.  The default
+			of -1 allows this to be automatically (and
+			dynamically) adjusted.	This parameter is intended
+			for use in testing.
+
 	rcupdate.rcu_task_ipi_delay= [KNL]
 			Set time in jiffies during which RCU tasks will
 			avoid sending IPIs, starting with the beginning
diff --git a/Documentation/admin-guide/kernel-per-CPU-kthreads.rst b/Documentation/admin-guide/kernel-per-CPU-kthreads.rst
index 5e51ee5b0358..e4a5fc26f1a9 100644
--- a/Documentation/admin-guide/kernel-per-CPU-kthreads.rst
+++ b/Documentation/admin-guide/kernel-per-CPU-kthreads.rst
@@ -208,7 +208,7 @@ Do at least one of the following:
 2.	Enable RCU to do its processing remotely via dyntick-idle by
 	doing all of the following:
 
-	a.	Build with CONFIG_NO_HZ=y and CONFIG_RCU_FAST_NO_HZ=y.
+	a.	Build with CONFIG_NO_HZ=y.
 	b.	Ensure that the CPU goes idle frequently, allowing other
 		CPUs to detect that it has passed through an RCU quiescent
 		state.	If the kernel is built with CONFIG_NO_HZ_FULL=y,
diff --git a/Documentation/timers/no_hz.rst b/Documentation/timers/no_hz.rst
index 20ad23a6c618..f8786be15183 100644
--- a/Documentation/timers/no_hz.rst
+++ b/Documentation/timers/no_hz.rst
@@ -184,16 +184,12 @@ There are situations in which idle CPUs cannot be permitted to
 enter either dyntick-idle mode or adaptive-tick mode, the most
 common being when that CPU has RCU callbacks pending.
 
-The CONFIG_RCU_FAST_NO_HZ=y Kconfig option may be used to cause such CPUs
-to enter dyntick-idle mode or adaptive-tick mode anyway.  In this case,
-a timer will awaken these CPUs every four jiffies in order to ensure
-that the RCU callbacks are processed in a timely fashion.
-
-Another approach is to offload RCU callback processing to "rcuo" kthreads
+Avoid this by offloading RCU callback processing to "rcuo" kthreads
 using the CONFIG_RCU_NOCB_CPU=y Kconfig option.  The specific CPUs to
 offload may be selected using The "rcu_nocbs=" kernel boot parameter,
 which takes a comma-separated list of CPUs and CPU ranges, for example,
-"1,3-5" selects CPUs 1, 3, 4, and 5.
+"1,3-5" selects CPUs 1, 3, 4, and 5.  Note that CPUs specified by
+the "nohz_full" kernel boot parameter are also offloaded.
 
 The offloaded CPUs will never queue RCU callbacks, and therefore RCU
 never prevents offloaded CPUs from entering either dyntick-idle mode