Messages in this thread Patch in this message | | | From | Valentin Schneider <> | Subject | Re: [PATCH v2] sched: Queue task on wakelist in the same llc if the wakee cpu is idle | Date | Mon, 30 May 2022 17:24:05 +0100 |
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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;
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