Re: [RFC PATCH] sched/fair: Choose the CPU where short task is running during wake up

[Honglei Wang]

Hi Prateek,


On 2022/9/30 01:34, K Prateek Nayak wrote:
> Hello Honglei,
>
> Thank you for looking into this.
>
> On 9/29/2022 12:29 PM, Honglei Wang wrote:
> > [..snip..]
> >
> > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > > > index 914096c5b1ae..7519ab5b911c 100644
> > > > > --- a/kernel/sched/fair.c
> > > > > +++ b/kernel/sched/fair.c
> > > > > @@ -6020,6 +6020,19 @@ static int wake_wide(struct task_struct *p)
> > > > >        return 1;
> > > > >    }
> > > > >    +/*
> > > > > + * If a task switches in and then voluntarily relinquishes the
> > > > > + * CPU quickly, it is regarded as a short running task.
> > > > > + * sysctl_sched_min_granularity is chosen as the threshold,
> > > > > + * as this value is the minimal slice if there are too many
> > > > > + * runnable tasks, see __sched_period().
> > > > > + */
> > > > > +static int is_short_task(struct task_struct *p)
> > > > > +{
> > > > > +    return (p->se.sum_exec_runtime <=
> > > > > +        (p->nvcsw * sysctl_sched_min_granularity));
> > > > > +}
> > > > > +
> > > > >    /*
> > > > >     * The purpose of wake_affine() is to quickly determine on which CPU we can run
> > > > >     * soonest. For the purpose of speed we only consider the waking and previous
> > > > > @@ -6050,7 +6063,8 @@ wake_affine_idle(int this_cpu, int prev_cpu, int sync)
> > > > >        if (available_idle_cpu(this_cpu) && cpus_share_cache(this_cpu, prev_cpu))
> > > > >            return available_idle_cpu(prev_cpu) ? prev_cpu : this_cpu;
> > > > >    -    if (sync && cpu_rq(this_cpu)->nr_running == 1)
> > > > > +    if ((sync && cpu_rq(this_cpu)->nr_running == 1) ||
> > > > > +        is_short_task(cpu_curr(this_cpu)))
> >
> > Seems it a bit breaks idle (or will be idle) purpose of wake_affine_idle() here. Maybe we can do it something like this?
> >
> > if ((sync || is_short_task(cpu_curr(this_cpu))) && cpu_rq(this_cpu)->nr_running == 1)
>
> I believe this will still cause performance degradation on split-LLC
> system for Stream like workloads. Based on the logs below, we can
> have a situation as follows:
>
> 	stream-4135    [029] d..2.   353.580957: select_task_rq_fair: wake_affine_idle: Select this_cpu: sync(0) rq->nr_running(1) is_short_task(1)
>
> Where sync is 0 but is_short_task() may return 1 and the
> current_rq->nr_running is 1. This will lead to two Stream threads
> getting placed on same LLC during wakeup which will cause cache
> contention and performance degradation.

What I meant was that we should not break the purpose of 
wake_affine_idle(). 'nr_running == 1' makes sure there won't be a long 
queue here, and this might be helpful in the benchmark tests as well. 
Probably the short code section I sent was not considerate.. It's just 
kinda clue.

I see your test result in another mail. It's great and is exactly what I 
was thinking we should test.

Thanks,
Honglei

> > Thanks,
> > Honglei
> >
> > > > This change seems to optimize for affine wakeup which benefits
> > > > tasks with producer-consumer pattern but is not ideal for Stream.
> > > > Currently the logic ends will do an affine wakeup even if sync
> > > > flag is not set:
> > > >
> > > >             stream-4135    [029] d..2.   353.580953: sched_waking: comm=stream pid=4129 prio=120 target_cpu=082
> > > >             stream-4135    [029] d..2.   353.580957: select_task_rq_fair: wake_affine_idle: Select this_cpu: sync(0) rq->nr_running(1) is_short_task(1)
> > > >             stream-4135    [029] d..2.   353.580960: sched_migrate_task: comm=stream pid=4129 prio=120 orig_cpu=82 dest_cpu=30
> > > >             <idle>-0       [030] dNh2.   353.580993: sched_wakeup: comm=stream pid=4129 prio=120 target_cpu=030
>
> This is the exact situation observed during our testing.
>
> > > > [..snip..]
> > > >   
> --
> Thanks and Regards,
> Prateek