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| author |   Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2018-07-25 11:25:23 -0700 |
|---|---|---|
| committer | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2018-08-30 16:03:43 -0700 |
| commit | c06aed0e31008a248c1841f1b7fc80e9ee242a31 (patch) | |
| tree | e66e83048dec7ef8b7c23444ab23f6e725021bd1 /Documentation/admin-guide | |
| parent | rcu: Provide functions for determining if call_rcu() has been invoked (diff) | |
| download | linux-dev-c06aed0e31008a248c1841f1b7fc80e9ee242a31.tar.xz linux-dev-c06aed0e31008a248c1841f1b7fc80e9ee242a31.zip | |
rcu: Compute jiffies_till_sched_qs from other kernel parameters
The jiffies_till_sched_qs value used to determine how old a grace period
must be before RCU enlists the help of the scheduler to force a quiescent
state on the holdout CPU. Currently, this defaults to HZ/10 regardless of
system size and may be set only at boot time. This can be a problem for
very large systems, because if the values of the jiffies_till_first_fqs
and jiffies_till_next_fqs kernel parameters are left at their defaults,
they are calculated to increase as the number of CPUs actually configured
on the system increases. Thus, on a sufficiently large system, RCU would
enlist the help of the scheduler before the grace-period kthread had a
chance to scan for idle CPUs, which wastes CPU time.
This commit therefore allows jiffies_till_sched_qs to be set, if desired,
but if left as default, computes is as jiffies_till_first_fqs plus twice
jiffies_till_next_fqs, thus allowing three force-quiescent-state scans
for idle CPUs. This scales with the number of CPUs, providing sensible
default values.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Diffstat (limited to 'Documentation/admin-guide')
| -rw-r--r-- | Documentation/admin-guide/kernel-parameters.txt | 9 |
1 files changed, 8 insertions, 1 deletions
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index aa96e669bcb8..6153fb62abe1 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -3595,7 +3595,14 @@ Set required age in jiffies for a given grace period before RCU starts soliciting quiescent-state help from - rcu_note_context_switch(). + rcu_note_context_switch(). If not specified, the + kernel will calculate a value based on the most + recent settings of rcutree.jiffies_till_first_fqs + and rcutree.jiffies_till_next_fqs. + This calculated value may be viewed in + rcutree.jiffies_to_sched_qs. Any attempt to + set rcutree.jiffies_to_sched_qs will be + cheerfully overwritten. rcutree.jiffies_till_first_fqs= [KNL] Set delay from grace-period initialization to |