Re: [PATCH v2] sched: Queue task on wakelist in the same llc if the wakee cpu is idle
From: Tianchen Ding
Date: Tue May 31 2022 - 11:38:43 EST
On 2022/5/31 21:55, Mel Gorman wrote:
On Tue, May 31, 2022 at 12:50:49PM +0100, Valentin Schneider wrote:
With all that in mind, I'm curious whether your patch is functionaly close
to the below.
---
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 66c4e5922fe1..ffd43264722a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3836,7 +3836,7 @@ static inline bool ttwu_queue_cond(int cpu, int wake_flags)
* the soon-to-be-idle CPU as the current CPU is likely busy.
* nr_running is checked to avoid unnecessary task stacking.
*/
- if ((wake_flags & WF_ON_CPU) && cpu_rq(cpu)->nr_running <= 1)
+ if (cpu_rq(cpu)->nr_running <= 1)
return true;
return false;
It's a little different. This may bring extra IPIs when nr_running == 1
and the current task on wakee cpu is not the target wakeup task (i.e.,
rq->curr == another_task && rq->curr != p). Then this another_task may
be disturbed by IPI which is not expected. So IMO the promise by
WF_ON_CPU is necessary.
You're right, actually taking a second look at that WF_ON_CPU path,
shouldn't the existing condition be:
if ((wake_flags & WF_ON_CPU) && !cpu_rq(cpu)->nr_running)
? Per the p->on_rq and p->on_cpu ordering, if we have WF_ON_CPU here then
we must have !p->on_rq, so the deactivate has happened, thus the task
being alone on the rq implies nr_running==0.
@Mel, do you remember why you went for <=1 here? I couldn't find any clues
on the original posting.
I don't recall exactly why I went with <= 1 there but I may not have
considered the memory ordering of on_rq and nr_running and the comment
above it is literally what I was thinking at the time. I think you're
right and that check can be !cpu_rq(cpu)->nr_running.
If the check becomes !cpu_rq(cpu)->nr_running
My patch would change, too.
Shall we remove WF_ON_CPU completely?