linux-dev/Documentation/RCU, branch linus/master

Merge branch 'exp.2022.05.11a' into HEAD

2022-05-11T18:49:35Z

exp.2022.05.11a: Expedited-grace-period latency-reduction updates.

rcu: Introduce CONFIG_RCU_EXP_CPU_STALL_TIMEOUT

2022-05-11T18:38:50Z

Currently both expedited and regular grace period stall warnings use a single timeout value that with units of seconds. However, recent Android use cases problem require a sub-100-millisecond expedited RCU CPU stall warning. Given that expedited RCU grace periods normally complete in far less than a single millisecond, especially for small systems, this is not unreasonable. Therefore introduce the CONFIG_RCU_EXP_CPU_STALL_TIMEOUT kernel configuration that defaults to 20 msec on Android and remains the same as that of the non-expedited stall warnings otherwise. It also can be changed in run-time via: /sys/.../parameters/rcu_exp_cpu_stall_timeout. [ paulmck: Default of zero to use CONFIG_RCU_STALL_TIMEOUT. ] Signed-off-by: Uladzislau Rezki Signed-off-by: Uladzislau Rezki (Sony) Signed-off-by: Paul E. McKenney

docs: Update RCU cross-references as suggested in doc-guide

2022-04-20T23:13:52Z

The RCU documentation contains old-style cross references which do not follow the best practices outlined in doc-guide/sphinx.rst. In addition, some of the cross references use URLs that should be replaced by pathnames. Update all of these cross references and adjust the surrounding words. Summary of changes: - out-of-date plaintext file names (*.txt) -> *.rst - references by :ref: tags -> path names of *.rst * use relative paths to .rst files under the RCU/ subdirectory * use abs paths of Documentation/xxx for other .rst files - references by URL under https://www.kernel.org/ -> paths of *.rst - adjust surrounding words of some of updated references. Note: The automarkup.py script interprets references via "*.txt" as if they were via "*.rst", so the *.txt -> *.rst changes should be regarded as cleanups rather than bug fixes. Cc: rcu@vger.kernel.org Cc: linux-doc@vger.kernel.org Tested-by: Bagas Sanjaya Reviewed-by: Steven Rostedt (Google) Signed-off-by: Akira Yokosawa Signed-off-by: Paul E. McKenney

docs: Add documentation for rude and trace RCU flavors

2022-04-20T22:57:02Z

This commit belatedly adds documentation of Tasks Rude RCU and Tasks Trace RCU to RCU's requirements document. Tested-by: Bagas Sanjaya Reviewed-by: Steven Rostedt (Google) Signed-off-by: Paul E. McKenney

Merge branches 'doc.2021.11.30c', 'exp.2021.12.07a', 'fastnohz.2021.11.30c', 'fixes.2021.11.30c', 'nocb.2021.12.09a', 'nolibc.2021.11.30c', 'tasks.2021.12.09a', 'torture.2021.12.07a' and 'torturescript.2021.11.30c' into HEAD

2021-12-09T19:38:09Z

doc.2021.11.30c: Documentation updates. exp.2021.12.07a: Expedited-grace-period fixes. fastnohz.2021.11.30c: Remove CONFIG_RCU_FAST_NO_HZ. fixes.2021.11.30c: Miscellaneous fixes. nocb.2021.12.09a: No-CB CPU updates. nolibc.2021.11.30c: Tiny in-kernel library updates. tasks.2021.12.09a: RCU-tasks updates, including update-side scalability. torture.2021.12.07a: Torture-test in-kernel module updates. torturescript.2021.11.30c: Torture-test scripting updates.

rcu: Remove the RCU_FAST_NO_HZ Kconfig option

2021-12-01T01:24:47Z

All of the uses of CONFIG_RCU_FAST_NO_HZ=y that I have seen involve systems with RCU callbacks offloaded. In this situation, all that this Kconfig option does is slow down idle entry/exit with an additional allways-taken early exit. If this is the only use case, then this Kconfig option nothing but an attractive nuisance that needs to go away. This commit therefore removes the RCU_FAST_NO_HZ Kconfig option. Signed-off-by: Paul E. McKenney

doc: RCU: Avoid 'Symbol' font-family in SVG figures

2021-12-01T01:23:14Z

On Ubuntu Focal, strings in some of SVG files under Documentation/RCU/Design can not be rendered properly when converted to PDF. Ubuntu releases since Focal and Debian bullseye have trouble with "Symbol" font-family in SVG files. As those strings are mostly API names such as "READ_ONCE()", "WRITE_ONCE(), "rcu_read_lock()", and so on, using a generic monospace font-family should be a good alternative. Substitute the font-family name by a simple sed pattern: 's/Symbol/monospace/g' Signed-off-by: Akira Yokosawa Signed-off-by: Paul E. McKenney

doc: Add refcount analogy to What is RCU

2021-12-01T01:23:06Z

The reader-writer-lock analogy is a useful way to think about RCU, but it is not always applicable. It is useful to have other analogies to work with, and particularly to emphasise that no single analogy is perfect. This patch add a "RCU as reference count" to the "what is RCU" document. See https://lwn.net/Articles/872559/ [ paulmck: Apply Akira Yokosawa feedback. ] Reviewed-by: Akira Yokosawa Signed-off-by: NeilBrown Signed-off-by: Paul E. McKenney

rcu: Fix undefined Kconfig macros

2021-09-13T23:32:46Z

Invoking scripts/checkkconfigsymbols.py in the Linux-kernel source tree located the following issues: 1. TREE_PREEMPT_RCU Referencing files: arch/sh/configs/sdk7786_defconfig It should now be CONFIG_PREEMPT_RCU. Except that the CONFIG_PREEMPT=y in that same file implies CONFIG_PREEMPT_RCU=y. Therefore, delete the CONFIG_TREE_PREEMPT_RCU=y line. The reason is as follows: In kernel/rcu/Kconfig, we have config PREEMPT_RCU bool default y if PREEMPTION https://www.kernel.org/doc/Documentation/kbuild/kconfig-language.txt says, "The default value is only assigned to the config symbol if no other value was set by the user (via the input prompt above)." there is no prompt in config PREEMPT_RCU entry, so we are guaranteed to get CONFIG_PREEMPT_RCU=y when CONFIG_PREEMPT is present. 2. RCU_CPU_STALL_INFO Referencing files: arch/xtensa/configs/nommu_kc705_defconfig The old Kconfig option RCU_CPU_STALL_INFO was removed by commit 75c27f119b64 ("rcu: Remove CONFIG_RCU_CPU_STALL_INFO"), and the kernel now acts as if this Kconfig option was unconditionally enabled. 3. RCU_NOCB_CPU_ALL Referencing files: Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst This is an old snapshot of the code. I update this from the real rcu_prepare_for_idle() function in kernel/rcu/tree_plugin.h. This change was tested by invoking "make htmldocs". 4. RCU_TORTURE_TESTS Referencing files: kernel/rcu/rcutorture.c Forward-progress checking conflicts with CPU-stall testing, so we should complain at "modprobe rcutorture" when both are enabled. Signed-off-by: Zhouyi Zhou Signed-off-by: Paul E. McKenney

doc: Add another stall-warning root cause in stallwarn.rst

2021-09-13T23:32:46Z

This commit adds a bullet item noting that both deficiencies and surpluses of calls to rcu_*_enter() and rcu_*_exit() can result in RCU CPU stall warnings. Signed-off-by: Paul E. McKenney