Messages in this thread | | | From | Suren Baghdasaryan <> | Date | Tue, 6 Jul 2021 19:42:37 -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 8:49 AM Suren Baghdasaryan <surenb@google.com> wrote: > > 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?).
To keep things moving I posted v3 (https://lore.kernel.org/patchwork/patch/1454547) with the changes I mentioned above. Let's keep discussing it there. Thanks!
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