diff options
| -rw-r--r-- | Documentation/scheduler/sched-bwc.rst | 74 | ||||
| -rw-r--r-- | kernel/sched/fair.c | 72 | ||||
| -rw-r--r-- | 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 3a9064219656..9801d6b284b1 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 fb75c0bea80f..7d8043fc8317 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4371,8 +4371,6 @@ 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) @@ -4394,8 +4392,7 @@ 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; @@ -4412,61 +4409,17 @@ 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; @@ -4661,8 +4614,7 @@ 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; @@ -4684,7 +4636,6 @@ 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) @@ -4709,7 +4660,7 @@ next: */ 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 */ @@ -4737,8 +4688,6 @@ 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 @@ -4751,8 +4700,7 @@ 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; @@ -4834,8 +4782,7 @@ 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 */ @@ -4868,7 +4815,6 @@ 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); @@ -4886,7 +4832,6 @@ 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; @@ -4895,11 +4840,10 @@ 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); } @@ -5056,8 +5000,6 @@ 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 7583faddba33..ea48aa5daeee 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; @@ -557,8 +555,6 @@ struct cfs_rq { #ifdef CONFIG_CFS_BANDWIDTH int runtime_enabled; - int expires_seq; - u64 runtime_expires; s64 runtime_remaining; u64 throttled_clock; |