Messages in this thread Patch in this message | | | From | Phil Auld <> | Subject | [PATCH v2] Sched/fair: Block nohz tick_stop when cfs bandwidth in use | Date | Tue, 27 Jun 2023 15:12:01 -0400 |
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CFS bandwidth limits and NOHZ full don't play well together. Tasks can easily run well past their quotas before a remote tick does accounting. This leads to long, multi-period stalls before such tasks can run again. Currentlyi, when presented with these conflicting requirements the scheduler is favoring nohz_full and letting the tick be stopped. However, nohz tick stopping is already best-effort, there are a number of conditions that can prevent it, whereas cfs runtime bandwidth is expected to be enforced.
Make the scheduler favor bandwidth over stopping the tick by setting TICK_DEP_BIT_SCHED when the only running task is a cfs task with runtime limit enabled.
Add sched_feat HZ_BW (off by default) to control this behavior.
Signed-off-by: Phil Auld <pauld@redhat.com> Cc: Ingo Molnar <mingo@redhat.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Vincent Guittot <vincent.guittot@linaro.org> Cc: Juri Lelli <juri.lelli@redhat.com> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com> Cc: Valentin Schneider <vschneid@redhat.com> Cc: Ben Segall <bsegall@google.com> ---
v2: Ben pointed out that the bit could get cleared in the dequeue path if we migrate a newly enqueued task without preempting curr. Added a check for that edge case to sched_can_stop_tick. Removed the call to sched_can_stop_tick from sched_fair_update_stop_tick since it was redundant.
kernel/sched/core.c | 12 +++++++++++ kernel/sched/fair.c | 45 +++++++++++++++++++++++++++++++++++++++++ kernel/sched/features.h | 2 ++ 3 files changed, 59 insertions(+)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index a68d1276bab0..646f60bfc7e7 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1194,6 +1194,8 @@ static void nohz_csd_func(void *info) #endif /* CONFIG_NO_HZ_COMMON */ #ifdef CONFIG_NO_HZ_FULL +extern bool sched_cfs_bandwidth_active(struct cfs_rq *cfs_rq); + bool sched_can_stop_tick(struct rq *rq) { int fifo_nr_running; @@ -1229,6 +1231,16 @@ bool sched_can_stop_tick(struct rq *rq) if (rq->nr_running > 1) return false; + /* + * If there is one task and it has CFS runtime bandwidth constraints + * and it's on the cpu now we don't want to stop the tick. + */ + if (sched_feat(HZ_BW) && rq->nr_running == 1 && rq->curr + && rq->curr->sched_class == &fair_sched_class && task_on_rq_queued(rq->curr)) { + if (sched_cfs_bandwidth_active(task_cfs_rq(rq->curr))) + return false; + } + return true; } #endif /* CONFIG_NO_HZ_FULL */ diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 373ff5f55884..a05af33b8da9 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6139,6 +6139,42 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq) rcu_read_unlock(); } +#ifdef CONFIG_NO_HZ_FULL + +bool sched_cfs_bandwidth_active(struct cfs_rq *cfs_rq) +{ + if (cfs_bandwidth_used() && cfs_rq->runtime_enabled) + return true; + + return false; +} + +/* called from pick_next_task_fair() */ +static void sched_fair_update_stop_tick(struct rq *rq, struct task_struct *p) +{ + struct cfs_rq *cfs_rq = task_cfs_rq(p); + int cpu = cpu_of(rq); + + if (!sched_feat(HZ_BW) || !cfs_bandwidth_used()) + return; + + if (!tick_nohz_full_cpu(cpu)) + return; + + if (rq->nr_running != 1) + return; + + /* + * We know there is only one task runnable and we've just picked it. The + * normal enqueue path will have cleared TICK_DEP_BIT_SCHED if we will + * be otherwise able to stop the tick. Just need to check if we are using + * bandwidth control. + */ + if (cfs_rq->runtime_enabled) + tick_nohz_dep_set_cpu(cpu, TICK_DEP_BIT_SCHED); +} +#endif + #else /* CONFIG_CFS_BANDWIDTH */ static inline bool cfs_bandwidth_used(void) @@ -6181,9 +6217,17 @@ static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg) static inline void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b) {} static inline void update_runtime_enabled(struct rq *rq) {} static inline void unthrottle_offline_cfs_rqs(struct rq *rq) {} +bool sched_cfs_bandwidth_active(struct cfs_rq *cfs_rq) +{ + return false; +} #endif /* CONFIG_CFS_BANDWIDTH */ +#if !defined(CONFIG_CFS_BANDWIDTH) || !defined(CONFIG_NO_HZ_FULL) +static inline void sched_fair_update_stop_tick(struct rq *rq, struct task_struct *p) {} +#endif + /************************************************** * CFS operations on tasks: */ @@ -8097,6 +8141,7 @@ done: __maybe_unused; hrtick_start_fair(rq, p); update_misfit_status(p, rq); + sched_fair_update_stop_tick(rq, p); return p; diff --git a/kernel/sched/features.h b/kernel/sched/features.h index ee7f23c76bd3..6fdf1fdf6b17 100644 --- a/kernel/sched/features.h +++ b/kernel/sched/features.h @@ -101,3 +101,5 @@ SCHED_FEAT(LATENCY_WARN, false) SCHED_FEAT(ALT_PERIOD, true) SCHED_FEAT(BASE_SLICE, true) + +SCHED_FEAT(HZ_BW, false) -- 2.31.1
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