Re: [PATCH 2/6] sched: add WF_CURRENT_CPU and externise ttwu

[Andrei Vagin]

On Fri, Apr 7, 2023 at 8:20 PM Chen Yu <yu.c.chen@xxxxxxxxx> wrote:
>
> On 2023-03-07 at 23:31:57 -0800, Andrei Vagin wrote:
> > From: Peter Oskolkov <posk@xxxxxxxxxx>
> >
> > Add WF_CURRENT_CPU wake flag that advices the scheduler to
> > move the wakee to the current CPU. This is useful for fast on-CPU
> > context switching use cases.
> >
> > In addition, make ttwu external rather than static so that
> > the flag could be passed to it from outside of sched/core.c.
> >
> > Signed-off-by: Peter Oskolkov <posk@xxxxxxxxxx>
> > Signed-off-by: Andrei Vagin <avagin@xxxxxxxxxx>
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -7569,6 +7569,10 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
> >       if (wake_flags & WF_TTWU) {
> >               record_wakee(p);
> >
> > +             if ((wake_flags & WF_CURRENT_CPU) &&
> > +                 cpumask_test_cpu(cpu, p->cpus_ptr))
> > +                     return cpu;
> > +
> I tried to reuse WF_CURRENT_CPU to mitigate the cross-cpu wakeup, however there
> are regressions when running some workloads, and these workloads want to be
> spreaded on idle CPUs whenever possible.
> The reason for the regression is that, above change chooses current CPU no matter
> what the load/utilization of this CPU is. So task are stacked on 1 CPU and hurts
> throughput/latency. And I believe this issue would be more severe on system with
> smaller number of CPU within 1 LLC(when compared to Intel platforms), such as AMD,
> Arm64.

WF_CURRENT_CPU works only in certain conditions. Maybe you saw my
attempt to change how WF_SYNC works:

https://www.spinics.net/lists/kernel/msg4567650.html

Then we've found that this idea doesn't work well, and it is a reason
why we have the separate WF_CURRENT_CPU flag.

>
> I know WF_CURRENT_CPU benefits seccomp, and can we make this change more genefic
> to benefit other workloads, by making the condition to trigger WF_CURRENT_CPU stricter?
> Say, only current CPU has 1 runnable task, and treat current CPU as the last resort by
> checking if the wakee's previous CPU is not idle. In this way, we can enable WF_CURRENT_CPU flag
> dynamically when some condition is met(a short task for example).

We discussed all of these here and here:

https://www.spinics.net/lists/kernel/msg4657545.html

https://lore.kernel.org/lkml/CANaxB-yWkKzhhPMGXCQbtjntJbqZ40FL2qtM2hk7LLWE-ZpbAg@xxxxxxxxxxxxxx/

I like your idea about short-duration tasks, but I think it is a
separate task and it has to be done in a separate patch set. Here, I
solve the problem of optimizing synchronous switches when one task wakes
up another one and falls asleep immediately after that. Waking up the
target task on the current CPU looks reasonable for a few reasons in
this case. First, waking up a task on the current CPU is cheaper than on
another one and it is much cheaper than waking on an idle cpu.  Second,
when tasks want to do synchronous switches, they often exchange some
data, so memory caches can play on us.

Thanks,
Andrei