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

[Chen Yu]

On 2023-04-09 at 21:56:26 -0700, Andrei Vagin wrote:
> 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 see, in seccomp case, even the idle CPU is not a better choice. 
> >
> > 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. 
It depends. For waker and wakee that compete for cache resource and do
not have share data, sometimes an idle target would be better.
> Second,
> when tasks want to do synchronous switches, they often exchange some
> data, so memory caches can play on us.
I like the name of 'WF_CURRENT_CPU' too : ) and I was thinking that if this could
become a auto-detect behavior so others can benefit from this.

If I understand correctly, the scenario this patch deals with is:
task A wakeups task B, task A and taks B have close relationship with each
other(cache sharing eg), when task A fall asleep, choose A's CPU, rather than an
idle CPU.

I'm thinking if the following logic would cover your case:
1. the waker A is a short duration one (waker will fall asleep soon)
2. the waker B is a short duration one (impact of B is minor)
3. the A->wakee_flips is 0 and A->last_wakee = B
4. the A->wakee_flips is 0 and B->last_wakee = A
5, cpu(A)->nr_running = 1

(3 and 4 mean that, A and B wake up each other, so it is likely that
they share cache data, and they are good firends to be put together)

If above conditions are met, choose current CPU. In this way, WF_CURRENT_CPU
can be set dynamically.

thanks,
Chenyu