This is a note to let you know that I've just added the patch titled
rcu: Avoid triggering strict-GP irq-work when RCU is idle
to the 5.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
rcu-avoid-triggering-strict-gp-irq-work-when-rcu-is-.patch
and it can be found in the queue-5.15 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 6e52e20495bd70094395b240998a71f4a9b9fb7f
Author: Zqiang <qiang1.zhang@xxxxxxxxx>
Date: Mon Aug 8 10:26:26 2022 +0800
rcu: Avoid triggering strict-GP irq-work when RCU is idle
[ Upstream commit 621189a1fe93cb2b34d62c5cdb9e258bca044813 ]
Kernels built with PREEMPT_RCU=y and RCU_STRICT_GRACE_PERIOD=y trigger
irq-work from rcu_read_unlock(), and the resulting irq-work handler
invokes rcu_preempt_deferred_qs_handle(). The point of this triggering
is to force grace periods to end quickly in order to give tools like KASAN
a better chance of detecting RCU usage bugs such as leaking RCU-protected
pointers out of an RCU read-side critical section.
However, this irq-work triggering is unconditional. This works, but
there is no point in doing this irq-work unless the current grace period
is waiting on the running CPU or task, which is not the common case.
After all, in the common case there are many rcu_read_unlock() calls
per CPU per grace period.
This commit therefore triggers the irq-work only when the current grace
period is waiting on the running CPU or task.
This change was tested as follows on a four-CPU system:
echo rcu_preempt_deferred_qs_handler > /sys/kernel/debug/tracing/set_ftrace_filter
echo 1 > /sys/kernel/debug/tracing/function_profile_enabled
insmod rcutorture.ko
sleep 20
rmmod rcutorture.ko
echo 0 > /sys/kernel/debug/tracing/function_profile_enabled
echo > /sys/kernel/debug/tracing/set_ftrace_filter
This procedure produces results in this per-CPU set of files:
/sys/kernel/debug/tracing/trace_stat/function*
Sample output from one of these files is as follows:
Function Hit Time Avg s^2
-------- --- ---- --- ---
rcu_preempt_deferred_qs_handle 838746 182650.3 us 0.217 us 0.004 us
The baseline sum of the "Hit" values (the number of calls to this
function) was 3,319,015. With this commit, that sum was 1,140,359,
for a 2.9x reduction. The worst-case variance across the CPUs was less
than 25%, so this large effect size is statistically significant.
The raw data is available in the Link: URL.
Link: https://lore.kernel.org/all/20220808022626.12825-1-qiang1.zhang@xxxxxxxxx/
Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index ef2dd131e955..f1a73a1f8472 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -638,7 +638,8 @@ static void rcu_read_unlock_special(struct task_struct *t)
expboost = (t->rcu_blocked_node && READ_ONCE(t->rcu_blocked_node->exp_tasks)) ||
(rdp->grpmask & READ_ONCE(rnp->expmask)) ||
- IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) ||
+ (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) &&
+ ((rdp->grpmask & READ_ONCE(rnp->qsmask)) || t->rcu_blocked_node)) ||
(IS_ENABLED(CONFIG_RCU_BOOST) && irqs_were_disabled &&
t->rcu_blocked_node);
// Need to defer quiescent state until everything is enabled.
