| Message ID | 20210707023933.1691149-1-surenb@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v3,1/1] psi: stop relying on timer_pending for poll_work rescheduling | expand |
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. > @@ -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. Thanks
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). */ > > > @@ -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() * * psi_group_change: * record_times() * smp_mb() * if xchg(poll_scheduled, 1) == 0 * mod_timer() * * atomic_xchg in psi_schedule_poll_work implements an implicit memory barrier but * we need an explicit one here. */ 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? > > Thanks
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. /* * 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. > * 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. 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. > 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
t 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
On Thu, Jul 08, 2021 at 08:54:56AM -0700, Suren Baghdasaryan wrote: > 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. Oh I see. Right, flipping the condition doesn't make sense because we need poll_scheduled to be set when we go ahead - whether we're forcing or not. I.e. if we were in a locked section, we'd write it like this: if (poll_scheduled) if (!force) return; else poll_scheduled = 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). > */ Personally, I still find this more confusing than no comment on !force, because when you read it it sort of raises the question what the alternatives would be. And the alternatives appear to be nonsensical code rather than legitimate options. But I won't insist if you prefer to leave it in. Your call. > > /* > > * 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? Yeah, sounds good to me!
On Thu, Jul 8, 2021 at 11:38 AM Johannes Weiner <hannes@cmpxchg.org> wrote: > > On Thu, Jul 08, 2021 at 08:54:56AM -0700, Suren Baghdasaryan wrote: > > 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. > > Oh I see. > > Right, flipping the condition doesn't make sense because we need > poll_scheduled to be set when we go ahead - whether we're forcing or > not. I.e. if we were in a locked section, we'd write it like this: > > if (poll_scheduled) > if (!force) > return; > else > poll_scheduled = 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). > > */ > > Personally, I still find this more confusing than no comment on > !force, because when you read it it sort of raises the question what > the alternatives would be. And the alternatives appear to be > nonsensical code rather than legitimate options. > > But I won't insist if you prefer to leave it in. Your call. I would like to keep it as a precaution, if you don't mind. In case someone in the future thinks about "optimizing" this by flipping the condition, hopefully the comment will give them a pause to think about it :) > > > > /* > > > * 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? > > Yeah, sounds good to me! Thanks! I'll post an update shortly.
On Thu, Jul 8, 2021 at 12:55 PM Suren Baghdasaryan <surenb@google.com> wrote: > > On Thu, Jul 8, 2021 at 11:38 AM Johannes Weiner <hannes@cmpxchg.org> wrote: > > > > On Thu, Jul 08, 2021 at 08:54:56AM -0700, Suren Baghdasaryan wrote: > > > 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. > > > > Oh I see. > > > > Right, flipping the condition doesn't make sense because we need > > poll_scheduled to be set when we go ahead - whether we're forcing or > > not. I.e. if we were in a locked section, we'd write it like this: > > > > if (poll_scheduled) > > if (!force) > > return; > > else > > poll_scheduled = 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). > > > */ > > > > Personally, I still find this more confusing than no comment on > > !force, because when you read it it sort of raises the question what > > the alternatives would be. And the alternatives appear to be > > nonsensical code rather than legitimate options. > > > > But I won't insist if you prefer to leave it in. Your call. > > I would like to keep it as a precaution, if you don't mind. In case > someone in the future thinks about "optimizing" this by flipping the > condition, hopefully the comment will give them a pause to think about > it :) > > > > > > > /* > > > > * 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? > > > > Yeah, sounds good to me! > > Thanks! I'll post an update shortly. v4 is posted at https://lore.kernel.org/patchwork/patch/1455172/
diff --git a/include/linux/psi_types.h b/include/linux/psi_types.h index 0a23300d49af..ef8bd89d065e 100644 --- a/include/linux/psi_types.h +++ b/include/linux/psi_types.h @@ -158,6 +158,7 @@ struct psi_group { struct timer_list poll_timer; wait_queue_head_t poll_wait; atomic_t poll_wakeup; + atomic_t poll_scheduled; /* Protects data used by the monitor */ struct mutex trigger_lock; diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c index 1652f2bb54b7..544676b2c1dc 100644 --- a/kernel/sched/psi.c +++ b/kernel/sched/psi.c @@ -196,6 +196,7 @@ static void group_init(struct psi_group *group) INIT_DELAYED_WORK(&group->avgs_work, psi_avgs_work); mutex_init(&group->avgs_lock); /* Init trigger-related members */ + atomic_set(&group->poll_scheduled, 0); mutex_init(&group->trigger_lock); INIT_LIST_HEAD(&group->triggers); memset(group->nr_triggers, 0, sizeof(group->nr_triggers)); @@ -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; rcu_read_lock(); @@ -582,12 +579,15 @@ static void psi_schedule_poll_work(struct psi_group *group, unsigned long delay) */ if (likely(task)) mod_timer(&group->poll_timer, jiffies + delay); + else + atomic_set(&group->poll_scheduled, 0); rcu_read_unlock(); } static void psi_poll_work(struct psi_group *group) { + bool force_reschedule = false; u32 changed_states; u64 now; @@ -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(); + } else { + /* Polling window is not over, keep rescheduling */ + force_reschedule = true; + } + + collect_percpu_times(group, PSI_POLL, &changed_states); if (changed_states & group->poll_states) { @@ -620,7 +642,8 @@ static void psi_poll_work(struct psi_group *group) group->polling_next_update = update_triggers(group, now); psi_schedule_poll_work(group, - nsecs_to_jiffies(group->polling_next_update - now) + 1); + nsecs_to_jiffies(group->polling_next_update - now) + 1, + force_reschedule); out: mutex_unlock(&group->trigger_lock); @@ -744,7 +767,7 @@ static void psi_group_change(struct psi_group *group, int cpu, write_seqcount_end(&groupc->seq); if (state_mask & group->poll_states) - psi_schedule_poll_work(group, 1); + psi_schedule_poll_work(group, 1, false); if (wake_clock && !delayed_work_pending(&group->avgs_work)) schedule_delayed_work(&group->avgs_work, PSI_FREQ); @@ -1239,6 +1262,7 @@ static void psi_trigger_destroy(struct kref *ref) * can no longer be found through group->poll_task. */ kthread_stop(task_to_destroy); + atomic_set(&group->poll_scheduled, 0); } kfree(t); }
Psi polling mechanism is trying to minimize the number of wakeups to run psi_poll_work and is currently relying on timer_pending() to detect when this work is already scheduled. This provides a window of opportunity for psi_group_change to schedule an immediate psi_poll_work after poll_timer_fn got called but before psi_poll_work could reschedule itself. Below is the depiction of this entire window: poll_timer_fn wake_up_interruptible(&group->poll_wait); psi_poll_worker wait_event_interruptible(group->poll_wait, ...) psi_poll_work psi_schedule_poll_work if (timer_pending(&group->poll_timer)) return; ... mod_timer(&group->poll_timer, jiffies + delay); Prior to 461daba06bdc we used to rely on poll_scheduled atomic which was reset and set back inside psi_poll_work and therefore this race window was much smaller. The larger window causes increased number of wakeups and our partners report visible power regression of ~10mA after applying 461daba06bdc. Bring back the poll_scheduled atomic and make this race window even narrower by resetting poll_scheduled only when we reach polling expiration time. This does not completely eliminate the possibility of extra wakeups caused by a race with psi_group_change however it will limit it to the worst case scenario of one extra wakeup per every tracking window (0.5s in the worst case). This patch also ensures correct ordering between clearing poll_scheduled flag and obtaining changed_states using memory barrier. Correct ordering between updating changed_states and setting poll_scheduled is ensured by atomic_xchg operation. By tracing the number of immediate rescheduling attempts performed by psi_group_change and the number of these attempts being blocked due to psi monitor being already active, we can assess the effects of this change: Before the patch: Run#1 Run#2 Run#3 Immediate reschedules attempted: 684365 1385156 1261240 Immediate reschedules blocked: 682846 1381654 1258682 Immediate reschedules (delta): 1519 3502 2558 Immediate reschedules (% of attempted): 0.22% 0.25% 0.20% After the patch: Run#1 Run#2 Run#3 Immediate reschedules attempted: 882244 770298 426218 Immediate reschedules blocked: 881996 769796 426074 Immediate reschedules (delta): 248 502 144 Immediate reschedules (% of attempted): 0.03% 0.07% 0.03% The number of non-blocked immediate reschedules dropped from 0.22-0.25% to 0.03-0.07%. The drop is attributed to the decrease in the race window size and the fact that we allow this race only when psi monitors reach polling window expiration time. Fixes: 461daba06bdc ("psi: eliminate kthread_worker from psi trigger scheduling mechanism") Reported-by: Kathleen Chang <yt.chang@mediatek.com> Reported-by: Wenju Xu <wenju.xu@mediatek.com> Reported-by: Jonathan Chen <jonathan.jmchen@mediatek.com> Signed-off-by: Suren Baghdasaryan <surenb@google.com> Tested-by: SH Chen <show-hong.chen@mediatek.com> --- - Replaced atomic_cmpxchg() with atomic_xchg() to ensure correct ordering per PeterZ - Added memory barrier between resetting poll_scheduled and obtaining changed_states per PeterZ and Johannes - Added a paragraph in the patch description about the ordering guarantees added in this patch include/linux/psi_types.h | 1 + kernel/sched/psi.c | 46 +++++++++++++++++++++++++++++---------- 2 files changed, 36 insertions(+), 11 deletions(-)