Messages in this thread | | | From | Suren Baghdasaryan <> | Date | Fri, 2 Jul 2021 08:49:59 -0700 | Subject | Re: [PATCH v2 1/1] psi: stop relying on timer_pending for poll_work rescheduling |
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On Fri, Jul 2, 2021 at 2:28 AM Peter Zijlstra <peterz@infradead.org> wrote: > > 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@infradead.org> 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@infradead.org> 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.
Oh, I see. That was my mistake. I was wrongly assuming that all RMW atomic operations are fully ordered but indeed, documentation states that: ``` - RMW operations that have a return value are fully ordered; - RMW operations that are conditional are unordered on FAILURE, otherwise the above rules apply. ``` So that's the actual functional difference here. Thanks for catching this and educating me!
> > 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).
Correct and I think I understand now the concern Johannes expressed.
> 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.
Got it. So I think the modifications needed to this patch is: 1. replacing atomic_cmpxchg(&group->poll_scheduled, 0, 1) with atomic_chg(&group->poll_scheduled, 1) 2. an explicit smp_mb() barrier right after atomic_set(&group->poll_scheduled, 0) in psi_poll_work().
I think that should ensure the correct ordering here. If you folks agree I'll respin v3 with these changes (or maybe I should respin and we continue discussion with that version?).
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