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

[Phil Auld <pauld@redhat.com>]

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
> 

--