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@xxxxxxxxxx>
> Reviewed-by: Ben Segall <bsegall@xxxxxxxxxx>
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@xxxxxxxxxx>.
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
>
--
