Re: [PATCH v2] sched: Queue task on wakelist in the same llc if the wakee cpu is idle

[Valentin Schneider <vschneid@redhat.com>]

On 27/05/22 17:05, Tianchen Ding wrote:
> The main idea of wakelist is to avoid cache bouncing. However,
> commit 518cd6234178 ("sched: Only queue remote wakeups when
> crossing cache boundaries") disabled queuing tasks on wakelist when
> the cpus share llc. This is because, at that time, the scheduler must
> send IPIs to do ttwu_queue_wakelist. Nowadays, ttwu_queue_wakelist also
> supports TIF_POLLING, so this is not a problem now when the wakee cpu is
> in idle polling.

[...]

> Our patch has improvement on schbench, hackbench
> and Pipe-based Context Switching of unixbench
> when there exists idle cpus,
> and no obvious regression on other tests of unixbench.
> This can help improve rt in scenes where wakeup happens frequently.
>
> Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>

This feels a bit like a generalization of

  2ebb17717550 ("sched/core: Offload wakee task activation if it the wakee is descheduling")

Given rq->curr is updated before prev->on_cpu is cleared, the waker
executing ttwu_queue_cond() can observe:

  p->on_rq=0
  p->on_cpu=1
  rq->curr=swapper/x (aka idle task)

So your addition of available_idle_cpu() in ttwu_queue_cond() (sort of)
matches that when invoked via:

        if (smp_load_acquire(&p->on_cpu) &&
            ttwu_queue_wakelist(p, task_cpu(p), wake_flags | WF_ON_CPU))
                goto unlock;

but it also affects

        ttwu_queue(p, cpu, wake_flags);

at the tail end of try_to_wake_up().

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;