Re: [PATCH v2 1/1] psi: stop relying on timer_pending for poll_work rescheduling

[Suren Baghdasaryan]

On Thu, Jul 1, 2021 at 9:12 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Thu, Jul 01, 2021 at 09:09:25AM -0700, Suren Baghdasaryan wrote:
> > On Thu, Jul 1, 2021 at 1:59 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > >
> > > On Wed, Jun 30, 2021 at 01:51:51PM -0700, Suren Baghdasaryan wrote:
> > > > +     /* cmpxchg should be called even when !force to set poll_scheduled */
> > > > +     if (atomic_cmpxchg(&group->poll_scheduled, 0, 1) && !force)
> > > >               return;
> > >
> > > Why is that a cmpxchg() ?
> >
> > We want to set poll_scheduled and proceed with rescheduling the timer
> > unless it's already scheduled, so cmpxchg helps us to make that
> > decision atomically. Or did I misunderstand your question?
>
> What's wrong with: atomic_xchg(&group->poll_scheduled, 1) ?

Yes, since poll_scheduled can be only 0 or 1 atomic_xchg should work
fine here. Functionally equivalent but I assume atomic_xchg() is more
efficient due to no comparison.
Will respin v3 if the rest looks ok.

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