3.2.91-rc1 review patch. If anyone has any objections, please let me know.
------------------
From: Matt Fleming <m...@codeblueprint.co.uk>
commit 6e5f32f7a43f45ee55c401c0b9585eb01f9629a8 upstream.
If we crossed a sample window while in NO_HZ we will add LOAD_FREQ to
the pending sample window time on exit, setting the next update not
one window into the future, but two.
This situation on exiting NO_HZ is described by:
this_rq->calc_load_update < jiffies < calc_load_update
In this scenario, what we should be doing is:
this_rq->calc_load_update = calc_load_update [ next window ]
But what we actually do is:
this_rq->calc_load_update = calc_load_update + LOAD_FREQ [ next+1 window ]
This has the effect of delaying load average updates for potentially
up to ~9seconds.
This can result in huge spikes in the load average values due to
per-cpu uninterruptible task counts being out of sync when accumulated
across all CPUs.
It's safe to update the per-cpu active count if we wake between sample
windows because any load that we left in 'calc_load_idle' will have
been zero'd when the idle load was folded in calc_global_load().
This issue is easy to reproduce before,
commit 9d89c257dfb9 ("sched/fair: Rewrite runnable load and utilization
average tracking")
just by forking short-lived process pipelines built from ps(1) and
grep(1) in a loop. I'm unable to reproduce the spikes after that
commit, but the bug still seems to be present from code review.
Signed-off-by: Matt Fleming <m...@codeblueprint.co.uk>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
Cc: Frederic Weisbecker <fweis...@gmail.com>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Mike Galbraith <efa...@gmx.de>
Cc: Mike Galbraith <umgwanakikb...@gmail.com>
Cc: Morten Rasmussen <morten.rasmus...@arm.com>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Vincent Guittot <vincent.guit...@linaro.org>
Fixes: commit 5167e8d ("sched/nohz: Rewrite and fix load-avg computation --
again")
Link: http://lkml.kernel.org/r/20170217120731.11868-2-m...@codeblueprint.co.uk
Signed-off-by: Ingo Molnar <mi...@kernel.org>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <b...@decadent.org.uk>
---
kernel/sched.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3617,8 +3617,9 @@ void calc_load_exit_idle(void)
struct rq *this_rq = this_rq();
/*
- * If we're still before the sample window, we're done.
+ * If we're still before the pending sample window, we're done.
*/
+ this_rq->calc_load_update = calc_load_update;
if (time_before(jiffies, this_rq->calc_load_update))
return;
@@ -3627,7 +3628,6 @@ void calc_load_exit_idle(void)
* accounted through the nohz accounting, so skip the entire deal and
* sync up for the next window.
*/
- this_rq->calc_load_update = calc_load_update;
if (time_before(jiffies, this_rq->calc_load_update + 10))
this_rq->calc_load_update += LOAD_FREQ;
}
