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

[Peter Zijlstra]

On Thu, Jul 01, 2021 at 09:28:04AM -0700, Suren Baghdasaryan wrote:
> 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.

Mostly conceptually simpler; the cmpxchg-on-0 makes that you have to
check if there's ever any state outside of {0,1}. The xchg() thing is
the classical test-and-set pattern.

On top of all that, the cmpxchg() can fail, which brings ordering
issues.

Typically, I think, you want to ensure that everything that happens
before psi_schedule_poll_work() is visible to the work when it runs
(also see Johannes' email). In case poll_scheduled is already 1, the
cmpxchg will fail and *NOT* provide that ordering. Meaning the work
might not observe the latest changes. xchg() doesn't have this subtlety.