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

[Tianchen Ding <dtcccc@linux.alibaba.com>]

On 2022/5/31 00:24, Valentin Schneider wrote:
> 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().

Yes. This part is what we mainly want to affect. The above WF_ON_CPU is 
not our point.

> 
> 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.