Re: [PATCH v6 1/1] sched/fair: Fix low cpu usage with high throttling by removing expiration of cpu-local slices

[Dave Chiluk <chiluk+linux@indeed.com>]

Thanks for all the help and testing you provided.  It's good to know
these changes have passed at least some scheduler regression tests.
If it comes to a v7 I'll add the Reviewed-by, otherwise I'll just let
Peter add it.

Will you be handling the backport into the RHEL 8 kernels?  I'll
submit this to Ubuntu and linux-stable once it gets accepted.

Thanks again,


On Tue, Jul 23, 2019 at 12:13 PM Phil Auld <pauld@redhat.com> wrote:
>
> Hi Dave,
>
> On Tue, Jul 23, 2019 at 11:44:26AM -0500 Dave Chiluk wrote:
> > It has been observed, that highly-threaded, non-cpu-bound applications
> > running under cpu.cfs_quota_us constraints can hit a high percentage of
> > periods throttled while simultaneously not consuming the allocated
> > amount of quota. This use case is typical of user-interactive non-cpu
> > bound applications, such as those running in kubernetes or mesos when
> > run on multiple cpu cores.
> >
> > This has been root caused to cpu-local run queue being allocated per cpu
> > bandwidth slices, and then not fully using that slice within the period.
> > At which point the slice and quota expires. This expiration of unused
> > slice results in applications not being able to utilize the quota for
> > which they are allocated.
> >
> > The non-expiration of per-cpu slices was recently fixed by
> > 'commit 512ac999d275 ("sched/fair: Fix bandwidth timer clock drift
> > condition")'. Prior to that it appears that this had been broken since
> > at least 'commit 51f2176d74ac ("sched/fair: Fix unlocked reads of some
> > cfs_b->quota/period")' which was introduced in v3.16-rc1 in 2014. That
> > added the following conditional which resulted in slices never being
> > expired.
> >
> > if (cfs_rq->runtime_expires != cfs_b->runtime_expires) {
> >       /* extend local deadline, drift is bounded above by 2 ticks */
> >       cfs_rq->runtime_expires += TICK_NSEC;
> >
> > Because this was broken for nearly 5 years, and has recently been fixed
> > and is now being noticed by many users running kubernetes
> > (https://github.com/kubernetes/kubernetes/issues/67577) it is my opinion
> > that the mechanisms around expiring runtime should be removed
> > altogether.
> >
> > This allows quota already allocated to per-cpu run-queues to live longer
> > than the period boundary. This allows threads on runqueues that do not
> > use much CPU to continue to use their remaining slice over a longer
> > period of time than cpu.cfs_period_us. However, this helps prevent the
> > above condition of hitting throttling while also not fully utilizing
> > your cpu quota.
> >
> > This theoretically allows a machine to use slightly more than its
> > allotted quota in some periods. This overflow would be bounded by the
> > remaining quota left on each per-cpu runqueueu. This is typically no
> > more than min_cfs_rq_runtime=1ms per cpu. For CPU bound tasks this will
> > change nothing, as they should theoretically fully utilize all of their
> > quota in each period. For user-interactive tasks as described above this
> > provides a much better user/application experience as their cpu
> > utilization will more closely match the amount they requested when they
> > hit throttling. This means that cpu limits no longer strictly apply per
> > period for non-cpu bound applications, but that they are still accurate
> > over longer timeframes.
> >
> > This greatly improves performance of high-thread-count, non-cpu bound
> > applications with low cfs_quota_us allocation on high-core-count
> > machines. In the case of an artificial testcase (10ms/100ms of quota on
> > 80 CPU machine), this commit resulted in almost 30x performance
> > improvement, while still maintaining correct cpu quota restrictions.
> > That testcase is available at https://github.com/indeedeng/fibtest.
> >
> > Fixes: 512ac999d275 ("sched/fair: Fix bandwidth timer clock drift condition")
> > Signed-off-by: Dave Chiluk <chiluk+linux@indeed.com>
> > Reviewed-by: Ben Segall <bsegall@google.com>
>
> This still works for me. The documentation reads pretty well, too. Good job.
>
> Feel free to add my Acked-by: or Reviewed-by: Phil Auld <pauld@redhat.com>.
>
> I'll run it through some more tests when I have time. The code is the same
> as the earlier one I tested from what I can see.
>
> Cheers,
> Phil
>
> > ---
> >  Documentation/scheduler/sched-bwc.rst | 74 ++++++++++++++++++++++++++++-------
> >  kernel/sched/fair.c                   | 72 ++++------------------------------
> >  kernel/sched/sched.h                  |  4 --
> >  3 files changed, 67 insertions(+), 83 deletions(-)
> >
> > diff --git a/Documentation/scheduler/sched-bwc.rst b/Documentation/scheduler/sched-bwc.rst
> > index 3a90642..9801d6b 100644
> > --- a/Documentation/scheduler/sched-bwc.rst
> > +++ b/Documentation/scheduler/sched-bwc.rst
> > @@ -9,15 +9,16 @@ CFS bandwidth control is a CONFIG_FAIR_GROUP_SCHED extension which allows the
> >  specification of the maximum CPU bandwidth available to a group or hierarchy.
> >
> >  The bandwidth allowed for a group is specified using a quota and period. Within
> > -each given "period" (microseconds), a group is allowed to consume only up to
> > -"quota" microseconds of CPU time.  When the CPU bandwidth consumption of a
> > -group exceeds this limit (for that period), the tasks belonging to its
> > -hierarchy will be throttled and are not allowed to run again until the next
> > -period.
> > -
> > -A group's unused runtime is globally tracked, being refreshed with quota units
> > -above at each period boundary.  As threads consume this bandwidth it is
> > -transferred to cpu-local "silos" on a demand basis.  The amount transferred
> > +each given "period" (microseconds), a task group is allocated up to "quota"
> > +microseconds of CPU time. That quota is assigned to per-cpu run queues in
> > +slices as threads in the cgroup become runnable. Once all quota has been
> > +assigned any additional requests for quota will result in those threads being
> > +throttled. Throttled threads will not be able to run again until the next
> > +period when the quota is replenished.
> > +
> > +A group's unassigned quota is globally tracked, being refreshed back to
> > +cfs_quota units at each period boundary. As threads consume this bandwidth it
> > +is transferred to cpu-local "silos" on a demand basis. The amount transferred
> >  within each of these updates is tunable and described as the "slice".
> >
> >  Management
> > @@ -35,12 +36,12 @@ The default values are::
> >
> >  A value of -1 for cpu.cfs_quota_us indicates that the group does not have any
> >  bandwidth restriction in place, such a group is described as an unconstrained
> > -bandwidth group.  This represents the traditional work-conserving behavior for
> > +bandwidth group. This represents the traditional work-conserving behavior for
> >  CFS.
> >
> >  Writing any (valid) positive value(s) will enact the specified bandwidth limit.
> > -The minimum quota allowed for the quota or period is 1ms.  There is also an
> > -upper bound on the period length of 1s.  Additional restrictions exist when
> > +The minimum quota allowed for the quota or period is 1ms. There is also an
> > +upper bound on the period length of 1s. Additional restrictions exist when
> >  bandwidth limits are used in a hierarchical fashion, these are explained in
> >  more detail below.
> >
> > @@ -53,8 +54,8 @@ unthrottled if it is in a constrained state.
> >  System wide settings
> >  --------------------
> >  For efficiency run-time is transferred between the global pool and CPU local
> > -"silos" in a batch fashion.  This greatly reduces global accounting pressure
> > -on large systems.  The amount transferred each time such an update is required
> > +"silos" in a batch fashion. This greatly reduces global accounting pressure
> > +on large systems. The amount transferred each time such an update is required
> >  is described as the "slice".
> >
> >  This is tunable via procfs::
> > @@ -97,6 +98,51 @@ There are two ways in which a group may become throttled:
> >  In case b) above, even though the child may have runtime remaining it will not
> >  be allowed to until the parent's runtime is refreshed.
> >
> > +CFS Bandwidth Quota Caveats
> > +---------------------------
> > +Once a slice is assigned to a cpu it does not expire.  However all but 1ms of
> > +the slice may be returned to the global pool if all threads on that cpu become
> > +unrunnable. This is configured at compile time by the min_cfs_rq_runtime
> > +variable. This is a performance tweak that helps prevent added contention on
> > +the global lock.
> > +
> > +The fact that cpu-local slices do not expire results in some interesting corner
> > +cases that should be understood.
> > +
> > +For cgroup cpu constrained applications that are cpu limited this is a
> > +relatively moot point because they will naturally consume the entirety of their
> > +quota as well as the entirety of each cpu-local slice in each period. As a
> > +result it is expected that nr_periods roughly equal nr_throttled, and that
> > +cpuacct.usage will increase roughly equal to cfs_quota_us in each period.
> > +
> > +For highly-threaded, non-cpu bound applications this non-expiration nuance
> > +allows applications to briefly burst past their quota limits by the amount of
> > +unused slice on each cpu that the task group is running on (typically at most
> > +1ms per cpu or as defined by min_cfs_rq_runtime).  This slight burst only
> > +applies if quota had been assigned to a cpu and then not fully used or returned
> > +in previous periods. This burst amount will not be transferred between cores.
> > +As a result, this mechanism still strictly limits the task group to quota
> > +average usage, albeit over a longer time window than a single period.  This
> > +also limits the burst ability to no more than 1ms per cpu.  This provides
> > +better more predictable user experience for highly threaded applications with
> > +small quota limits on high core count machines. It also eliminates the
> > +propensity to throttle these applications while simultanously using less than
> > +quota amounts of cpu. Another way to say this, is that by allowing the unused
> > +portion of a slice to remain valid across periods we have decreased the
> > +possibility of wastefully expiring quota on cpu-local silos that don't need a
> > +full slice's amount of cpu time.
> > +
> > +The interaction between cpu-bound and non-cpu-bound-interactive applications
> > +should also be considered, especially when single core usage hits 100%. If you
> > +gave each of these applications half of a cpu-core and they both got scheduled
> > +on the same CPU it is theoretically possible that the non-cpu bound application
> > +will use up to 1ms additional quota in some periods, thereby preventing the
> > +cpu-bound application from fully using its quota by that same amount. In these
> > +instances it will be up to the CFS algorithm (see sched-design-CFS.rst) to
> > +decide which application is chosen to run, as they will both be runnable and
> > +have remaining quota. This runtime discrepancy will be made up in the following
> > +periods when the interactive application idles.
> > +
> >  Examples
> >  --------
> >  1. Limit a group to 1 CPU worth of runtime::
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 036be95..00b68f0 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -4316,8 +4316,6 @@ void __refill_cfs_bandwidth_runtime(struct cfs_bandwidth *cfs_b)
> >
> >       now = sched_clock_cpu(smp_processor_id());
> >       cfs_b->runtime = cfs_b->quota;
> > -     cfs_b->runtime_expires = now + ktime_to_ns(cfs_b->period);
> > -     cfs_b->expires_seq++;
> >  }
> >
> >  static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
> > @@ -4339,8 +4337,7 @@ static int assign_cfs_rq_runtime(struct cfs_rq *cfs_rq)
> >  {
> >       struct task_group *tg = cfs_rq->tg;
> >       struct cfs_bandwidth *cfs_b = tg_cfs_bandwidth(tg);
> > -     u64 amount = 0, min_amount, expires;
> > -     int expires_seq;
> > +     u64 amount = 0, min_amount;
> >
> >       /* note: this is a positive sum as runtime_remaining <= 0 */
> >       min_amount = sched_cfs_bandwidth_slice() - cfs_rq->runtime_remaining;
> > @@ -4357,61 +4354,17 @@ static int assign_cfs_rq_runtime(struct cfs_rq *cfs_rq)
> >                       cfs_b->idle = 0;
> >               }
> >       }
> > -     expires_seq = cfs_b->expires_seq;
> > -     expires = cfs_b->runtime_expires;
> >       raw_spin_unlock(&cfs_b->lock);
> >
> >       cfs_rq->runtime_remaining += amount;
> > -     /*
> > -      * we may have advanced our local expiration to account for allowed
> > -      * spread between our sched_clock and the one on which runtime was
> > -      * issued.
> > -      */
> > -     if (cfs_rq->expires_seq != expires_seq) {
> > -             cfs_rq->expires_seq = expires_seq;
> > -             cfs_rq->runtime_expires = expires;
> > -     }
> >
> >       return cfs_rq->runtime_remaining > 0;
> >  }
> >
> > -/*
> > - * Note: This depends on the synchronization provided by sched_clock and the
> > - * fact that rq->clock snapshots this value.
> > - */
> > -static void expire_cfs_rq_runtime(struct cfs_rq *cfs_rq)
> > -{
> > -     struct cfs_bandwidth *cfs_b = tg_cfs_bandwidth(cfs_rq->tg);
> > -
> > -     /* if the deadline is ahead of our clock, nothing to do */
> > -     if (likely((s64)(rq_clock(rq_of(cfs_rq)) - cfs_rq->runtime_expires) < 0))
> > -             return;
> > -
> > -     if (cfs_rq->runtime_remaining < 0)
> > -             return;
> > -
> > -     /*
> > -      * If the local deadline has passed we have to consider the
> > -      * possibility that our sched_clock is 'fast' and the global deadline
> > -      * has not truly expired.
> > -      *
> > -      * Fortunately we can check determine whether this the case by checking
> > -      * whether the global deadline(cfs_b->expires_seq) has advanced.
> > -      */
> > -     if (cfs_rq->expires_seq == cfs_b->expires_seq) {
> > -             /* extend local deadline, drift is bounded above by 2 ticks */
> > -             cfs_rq->runtime_expires += TICK_NSEC;
> > -     } else {
> > -             /* global deadline is ahead, expiration has passed */
> > -             cfs_rq->runtime_remaining = 0;
> > -     }
> > -}
> > -
> >  static void __account_cfs_rq_runtime(struct cfs_rq *cfs_rq, u64 delta_exec)
> >  {
> >       /* dock delta_exec before expiring quota (as it could span periods) */
> >       cfs_rq->runtime_remaining -= delta_exec;
> > -     expire_cfs_rq_runtime(cfs_rq);
> >
> >       if (likely(cfs_rq->runtime_remaining > 0))
> >               return;
> > @@ -4602,8 +4555,7 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
> >               resched_curr(rq);
> >  }
> >
> > -static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b,
> > -             u64 remaining, u64 expires)
> > +static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b, u64 remaining)
> >  {
> >       struct cfs_rq *cfs_rq;
> >       u64 runtime;
> > @@ -4625,7 +4577,6 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b,
> >               remaining -= runtime;
> >
> >               cfs_rq->runtime_remaining += runtime;
> > -             cfs_rq->runtime_expires = expires;
> >
> >               /* we check whether we're throttled above */
> >               if (cfs_rq->runtime_remaining > 0)
> > @@ -4650,7 +4601,7 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b,
> >   */
> >  static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun, unsigned long flags)
> >  {
> > -     u64 runtime, runtime_expires;
> > +     u64 runtime;
> >       int throttled;
> >
> >       /* no need to continue the timer with no bandwidth constraint */
> > @@ -4678,8 +4629,6 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun, u
> >       /* account preceding periods in which throttling occurred */
> >       cfs_b->nr_throttled += overrun;
> >
> > -     runtime_expires = cfs_b->runtime_expires;
> > -
> >       /*
> >        * This check is repeated as we are holding onto the new bandwidth while
> >        * we unthrottle. This can potentially race with an unthrottled group
> > @@ -4692,8 +4641,7 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun, u
> >               cfs_b->distribute_running = 1;
> >               raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
> >               /* we can't nest cfs_b->lock while distributing bandwidth */
> > -             runtime = distribute_cfs_runtime(cfs_b, runtime,
> > -                                              runtime_expires);
> > +             runtime = distribute_cfs_runtime(cfs_b, runtime);
> >               raw_spin_lock_irqsave(&cfs_b->lock, flags);
> >
> >               cfs_b->distribute_running = 0;
> > @@ -4775,8 +4723,7 @@ static void __return_cfs_rq_runtime(struct cfs_rq *cfs_rq)
> >               return;
> >
> >       raw_spin_lock(&cfs_b->lock);
> > -     if (cfs_b->quota != RUNTIME_INF &&
> > -         cfs_rq->runtime_expires == cfs_b->runtime_expires) {
> > +     if (cfs_b->quota != RUNTIME_INF) {
> >               cfs_b->runtime += slack_runtime;
> >
> >               /* we are under rq->lock, defer unthrottling using a timer */
> > @@ -4809,7 +4756,6 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
> >  {
> >       u64 runtime = 0, slice = sched_cfs_bandwidth_slice();
> >       unsigned long flags;
> > -     u64 expires;
> >
> >       /* confirm we're still not at a refresh boundary */
> >       raw_spin_lock_irqsave(&cfs_b->lock, flags);
> > @@ -4827,7 +4773,6 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
> >       if (cfs_b->quota != RUNTIME_INF && cfs_b->runtime > slice)
> >               runtime = cfs_b->runtime;
> >
> > -     expires = cfs_b->runtime_expires;
> >       if (runtime)
> >               cfs_b->distribute_running = 1;
> >
> > @@ -4836,11 +4781,10 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
> >       if (!runtime)
> >               return;
> >
> > -     runtime = distribute_cfs_runtime(cfs_b, runtime, expires);
> > +     runtime = distribute_cfs_runtime(cfs_b, runtime);
> >
> >       raw_spin_lock_irqsave(&cfs_b->lock, flags);
> > -     if (expires == cfs_b->runtime_expires)
> > -             lsub_positive(&cfs_b->runtime, runtime);
> > +     lsub_positive(&cfs_b->runtime, runtime);
> >       cfs_b->distribute_running = 0;
> >       raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
> >  }
> > @@ -4997,8 +4941,6 @@ void start_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
> >
> >       cfs_b->period_active = 1;
> >       overrun = hrtimer_forward_now(&cfs_b->period_timer, cfs_b->period);
> > -     cfs_b->runtime_expires += (overrun + 1) * ktime_to_ns(cfs_b->period);
> > -     cfs_b->expires_seq++;
> >       hrtimer_start_expires(&cfs_b->period_timer, HRTIMER_MODE_ABS_PINNED);
> >  }
> >
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index 802b1f3..28c16e9 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -335,8 +335,6 @@ struct cfs_bandwidth {
> >       u64                     quota;
> >       u64                     runtime;
> >       s64                     hierarchical_quota;
> > -     u64                     runtime_expires;
> > -     int                     expires_seq;
> >
> >       u8                      idle;
> >       u8                      period_active;
> > @@ -556,8 +554,6 @@ struct cfs_rq {
> >
> >  #ifdef CONFIG_CFS_BANDWIDTH
> >       int                     runtime_enabled;
> > -     int                     expires_seq;
> > -     u64                     runtime_expires;
> >       s64                     runtime_remaining;
> >
> >       u64                     throttled_clock;
> > --
> > 1.8.3.1
> >
>
> --