Changes representative of linux-3.10.0-1062.9.1.el7.tar.xz

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
@@ -33,12 +34,12 @@ 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.
 
@@ -51,8 +52,8 @@ 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:
@@ -90,6 +91,51 @@ 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.txt) 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.

Makefile

@@ -5,7 +5,7 @@ EXTRAVERSION =
 NAME = Unicycling Gorilla
 RHEL_MAJOR = 7
 RHEL_MINOR = 7
-RHEL_RELEASE = 1062.7.1
+RHEL_RELEASE = 1062.9.1
 
 #
 # DRM backport version

kernel/sched/core.c

@@ -9450,6 +9450,11 @@ static void cpu_cgroup_css_offline(struct cgroup *cgrp)
 	sched_offline_group(tg);
 }
 
+static void cpu_cgroup_fork(struct task_struct *task, void *private)
+{
+	sched_move_task(task);
+}
+
 static int cpu_cgroup_can_attach(struct cgroup *cgrp,
 				 struct cgroup_taskset *tset)
 {
@@ -9813,6 +9818,7 @@ struct cgroup_subsys cpu_cgroup_subsys = {
 	.css_free	= cpu_cgroup_css_free,
 	.css_online	= cpu_cgroup_css_online,
 	.css_offline	= cpu_cgroup_css_offline,
+	.fork		= cpu_cgroup_fork,
 	.can_attach	= cpu_cgroup_can_attach,
 	.attach		= cpu_cgroup_attach,
 	.exit		= cpu_cgroup_exit,

kernel/sched/fair.c

@@ -3225,8 +3225,6 @@ 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)
@@ -3248,8 +3246,7 @@ 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;
@@ -3275,61 +3272,17 @@ 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;
@@ -3504,8 +3457,7 @@ 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;
@@ -3526,7 +3478,6 @@ 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)
@@ -3551,7 +3502,7 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b,
  */
 static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
 {
-	u64 runtime, runtime_expires;
+	u64 runtime;
 	int throttled;
 
 	/* no need to continue the timer with no bandwidth constraint */
@@ -3586,8 +3537,6 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
 	/* 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
@@ -3600,8 +3549,7 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
 		cfs_b->distribute_running = 1;
 		raw_spin_unlock(&cfs_b->lock);
 		/* 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(&cfs_b->lock);
 
 		cfs_b->distribute_running = 0;
@@ -3678,8 +3626,7 @@ 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 */
@@ -3711,7 +3658,6 @@ static __always_inline void return_cfs_rq_runtime(struct cfs_rq *cfs_rq)
 static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
 {
 	u64 runtime = 0, slice = sched_cfs_bandwidth_slice();
-	u64 expires;
 
 	/* confirm we're still not at a refresh boundary */
 	raw_spin_lock(&cfs_b->lock);
@@ -3728,7 +3674,6 @@ 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;
 
@@ -3737,11 +3682,10 @@ 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(&cfs_b->lock);
-	if (expires == cfs_b->runtime_expires)
-		cfs_b->runtime -= min(runtime, cfs_b->runtime);
+	cfs_b->runtime -= min(runtime, cfs_b->runtime);
 	cfs_b->distribute_running = 0;
 	raw_spin_unlock(&cfs_b->lock);
 }

kernel/sched/sched.h

@@ -232,14 +232,10 @@ struct cfs_bandwidth {
 	ktime_t period;
 	u64 quota, runtime;
 	s64 hierarchal_quota;
-	u64 runtime_expires;
-#ifndef __GENKSYMS__
-	int expires_seq;
-	short idle;
-	short timer_active;
-#else
+
+	RH_KABI_DEPRECATE(u64, runtime_expires)
 	int idle, timer_active;
-#endif
+
 	struct hrtimer period_timer, slack_timer;
 	struct list_head throttled_cfs_rq;
 
@@ -440,7 +436,7 @@ struct cfs_rq {
 
 #ifdef CONFIG_CFS_BANDWIDTH
 	int runtime_enabled;
-	u64 runtime_expires;
+	RH_KABI_DEPRECATE(u64, runtime_expires)
 	s64 runtime_remaining;
 
 	u64 throttled_clock, throttled_clock_task;
@@ -452,9 +448,6 @@ struct cfs_rq {
 	RH_KABI_EXTEND(u64 last_h_load_update)
 	RH_KABI_EXTEND(struct sched_entity *h_load_next)
 #endif
-#ifdef CONFIG_CFS_BANDWIDTH
-	RH_KABI_EXTEND(int expires_seq)
-#endif
 #endif /* CONFIG_FAIR_GROUP_SCHED */
 };