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From:   Dave Chiluk <chiluk+linux@indeed.com>
To:     Ben Segall <bsegall@google.com>, Phil Auld <pauld@redhat.com>,
        Peter Oskolkov <posk@posk.io>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, Brendan Gregg <bgregg@netflix.com>,
        Kyle Anderson <kwa@yelp.com>,
        Gabriel Munos <gmunoz@netflix.com>,
        John Hammond <jhammond@indeed.com>,
        Cong Wang <xiyou.wangcong@gmail.com>,
        Jonathan Corbet <corbet@lwn.net>, linux-doc@vger.kernel.org
Subject: [PATCH v3 1/1] sched/fair: Fix low cpu usage with high throttling by removing expiration of cpu-local slices
Date:   Wed, 29 May 2019 14:08:46 -0500
Message-Id: <1559156926-31336-2-git-send-email-chiluk+linux@indeed.com>
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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 threads 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 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 has 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 only per-cpu slices 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 prevents the above condition of
hitting throttling while also not fully utilizing your cpu quota.

This theoretically allows a machine to use slightly more than it's
allotted quota in some periods.  This overflow would be bounded by the
remaining per-cpu slice that was left un-used in the previous period.
For CPU bound tasks this will change nothing, as they should
theoretically fully utilize all of their quota and slices 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 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, this performance
discrepancy has been observed to be almost 30x performance improvement,
while still maintaining correct cpu quota restrictions albeit over
longer time intervals than cpu.cfs_period_us.  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>
---
 Documentation/scheduler/sched-bwc.txt | 56 ++++++++++++++++++++++-----
 kernel/sched/fair.c                   | 71 +++--------------------------------
 kernel/sched/sched.h                  |  4 --
 3 files changed, 53 insertions(+), 78 deletions(-)

diff --git a/Documentation/scheduler/sched-bwc.txt b/Documentation/scheduler/sched-bwc.txt
index f6b1873..260fd65 100644
--- a/Documentation/scheduler/sched-bwc.txt
+++ b/Documentation/scheduler/sched-bwc.txt
@@ -8,15 +8,16 @@ 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
@@ -90,6 +91,43 @@ 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.
 
+Real-world behavior of slice non-expiration
+-------------------------------------------
+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.
+
+However in a worst-case scenario, highly-threaded, interactive/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.  This slight burst requires that quota had been assigned
+and then not fully used 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
+period.  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 wasting 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 sched_cfs_bandwidth_slice_us additional quota in some periods,
+thereby preventing the cpu-bound application from fully using it's quota by
+that same amount.  In these instances it will be up to the CFS algorithm (see
+sched-design-CFS.txt) to decide which application is chosen to run, as they
+will both be runnable and have remaining quota.  This runtime discrepancy will
+should 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 f35930f..a675c69 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4295,8 +4295,6 @@ 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)
@@ -4318,8 +4316,7 @@ 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;
@@ -4336,61 +4333,17 @@ 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;
@@ -4581,8 +4534,7 @@ 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;
@@ -4604,7 +4556,6 @@ 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)
@@ -4629,7 +4580,7 @@ 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 */
@@ -4657,8 +4608,6 @@ 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
@@ -4671,8 +4620,7 @@ 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;
@@ -4749,8 +4697,7 @@ 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 */
@@ -4783,7 +4730,6 @@ 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);
@@ -4800,7 +4746,6 @@ 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;
 
@@ -4809,11 +4754,9 @@ 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);
 	cfs_b->distribute_running = 0;
 	raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
 }
@@ -4969,8 +4912,6 @@ 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 b52ed1a..0c0ed23 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -341,8 +341,6 @@ struct cfs_bandwidth {
 	u64			quota;
 	u64			runtime;
 	s64			hierarchical_quota;
-	u64			runtime_expires;
-	int			expires_seq;
 
 	short			idle;
 	short			period_active;
@@ -562,8 +560,6 @@ 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