Messages in this thread Patch in this message | | | From | Suren Baghdasaryan <> | Date | Thu, 8 Jul 2021 08:54:56 -0700 | Subject | Re: [PATCH v3 1/1] psi: stop relying on timer_pending for poll_work rescheduling |
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On Thu, Jul 8, 2021 at 7:44 AM Johannes Weiner <hannes@cmpxchg.org> wrote: > > On Wed, Jul 07, 2021 at 03:43:48PM -0700, Suren Baghdasaryan wrote: > > On Wed, Jul 7, 2021 at 6:39 AM Johannes Weiner <hannes@cmpxchg.org> wrote: > > > > > > This looks good to me now code wise. Just a comment on the comments: > > > > > > On Tue, Jul 06, 2021 at 07:39:33PM -0700, Suren Baghdasaryan wrote: > > > > @@ -559,18 +560,14 @@ static u64 update_triggers(struct psi_group *group, u64 now) > > > > return now + group->poll_min_period; > > > > } > > > > > > > > -/* Schedule polling if it's not already scheduled. */ > > > > -static void psi_schedule_poll_work(struct psi_group *group, unsigned long delay) > > > > +/* Schedule polling if it's not already scheduled or forced. */ > > > > +static void psi_schedule_poll_work(struct psi_group *group, unsigned long delay, > > > > + bool force) > > > > { > > > > struct task_struct *task; > > > > > > > > - /* > > > > - * Do not reschedule if already scheduled. > > > > - * Possible race with a timer scheduled after this check but before > > > > - * mod_timer below can be tolerated because group->polling_next_update > > > > - * will keep updates on schedule. > > > > - */ > > > > - if (timer_pending(&group->poll_timer)) > > > > + /* xchg should be called even when !force to set poll_scheduled */ > > > > + if (atomic_xchg(&group->poll_scheduled, 1) && !force) > > > > return; > > > > > > This explains what the code does, but not why. It would be good to > > > explain the ordering with poll_work, here or there. But both sides > > > should mention each other. > > > > How about this: > > > > /* > > * atomic_xchg should be called even when !force to always set poll_scheduled > > * and to provide a memory barrier (see the comment inside psi_poll_work). > > */ > > The memory barrier part makes sense, but the first part says what the > code does and the message is unclear to me. Are you worried somebody > might turn this around in the future and only conditionalize on > poll_scheduled when !force? Essentially, I don't see the downside of > dropping that. But maybe I'm missing something.
Actually you are right. Originally I was worried that there might be a case when poll_scheduled==0 and force==true and if someone flips the conditions we will reschedule the timer but will not set poll_scheduled back to 1. However I don't think this condition is possible. We set force=true only when we skipped resetting poll_schedule to 0 and on initial wakeup we always reset poll_schedule. How about changing the comment to this:
/* * atomic_xchg should be called even when !force to provide a * full memory barrier (see the comment inside psi_poll_work). */
> /* > * The xchg implies a full barrier that matches the one > * in psi_poll_work() (see corresponding comment there). > */ > > > > > @@ -595,6 +595,28 @@ static void psi_poll_work(struct psi_group *group) > > > > > > > > now = sched_clock(); > > > > > > > > + if (now > group->polling_until) { > > > > + /* > > > > + * We are either about to start or might stop polling if no > > > > + * state change was recorded. Resetting poll_scheduled leaves > > > > + * a small window for psi_group_change to sneak in and schedule > > > > + * an immegiate poll_work before we get to rescheduling. One > > > > + * potential extra wakeup at the end of the polling window > > > > + * should be negligible and polling_next_update still keeps > > > > + * updates correctly on schedule. > > > > + */ > > > > + atomic_set(&group->poll_scheduled, 0); > > > > + /* > > > > + * Ensure that operations of clearing group->poll_scheduled and > > > > + * obtaining changed_states are not reordered. > > > > + */ > > > > + smp_mb(); > > > > > > Same here, it would be good to explain that this is ordering the > > > scheduler with the timer such that no events are missed. Feel free to > > > reuse my race diagram from the other thread - those are better at > > > conveying the situation than freeform text. > > > > I tried to make your diagram a bit less abstract by using the actual > > names. How about this? > > > > /* > > * We need to enforce ordering between poll_scheduled and psi_group_cpu.times > > * reads and writes in psi_poll_work and psi_group_change functions. > > Otherwise we > > * might fail to reschedule the timer when monitored states change: > > * > > * psi_poll_work: > > * poll_scheduled = 0 > > * smp_mb() > > * changed_states = collect_percpu_times() > > * if changed_states && xchg(poll_scheduled, 1) == 0 > > * mod_timer() > > Those last two lines aren't relevant for the race, right? I'd leave > those out to not distract from it.
They did help me illustrate the two failure cases but yeah, someone who can read the code can derive the rest :)
> > > * psi_group_change: > > * record_times() > > * smp_mb() > > * if xchg(poll_scheduled, 1) == 0 > > * mod_timer() > > The reason I tend to keep these more abstract is because 1) the names > of the functions change (I had already sent out patches to rename half > the variable and function names in this diagram), while the > architecture (task change vs poll worker) likely won't, and 2) because > it's easy to drown out what the reads, writes, and thus the race > condition is with code details and function call indirections.
Got it.
> > How about a compromise? > > /* > * A task change can race with the poll worker that is supposed to > * report on it. To avoid missing events, ensure ordering between > * poll_scheduled and the task state accesses, such that if the poll > * worker misses the state update, the task change is guaranteed to > * reschedule the poll worker: > * > * poll worker: > * atomic_set(poll_scheduled, 0) > * smp_mb() > * LOAD states > * > * task change: > * STORE states > * if atomic_xchg(poll_scheduled, 1) == 0: > * schedule poll worker > * > * The atomic_xchg() implies a full barrier. > */ > smp_mb(); > > This gives a high-level view of what's happening but it can still be > mapped to the code by following the poll_scheduled variable.
This looks really good to me. If you agree on the first comment modification, should I respin the next version?
> > If we remove smp_mb barriers then there are the following possible > > reordering cases: > > > > Case1: reordering in psi_poll_work > > psi_poll_work psi_group_change > > changed_states = collect_percpu_times() > > record_times() > > if xchg(poll_scheduled, > > 1) == 0 <-- false > > mod_timer() > > poll_scheduled = 0 > > if changed_states && xchg(poll_scheduled, 1) == 0 <-- changed_states is false > > mod_timer() > > > > Case2: reordering in psi_group_change > > psi_poll_work psi_group_change > > if xchg(poll_scheduled, > > 1) == 0 <-- false > > mod_timer() > > poll_scheduled = 0 > > changed_states = collect_percpu_times() > > record_times() > > if changed_states && xchg(poll_scheduled, 1) == 0 <-- changed_states is false > > mod_timer() > > > > In both cases mod_timer() is not called, poll update is missed. But > > describing this all in the comments would be an overkill IMHO. > > WDYT? > > Yeah, I also think that's overkill. The failure cases can be derived > from the concurrency diagram and explanation. > > Thanks
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