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Date: Fri, 16 Sep 2022 15:15:38 +0200 From: Vincent Guittot <vincent.guittot@...aro.org> To: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com, dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com, linux-kernel@...r.kernel.org Cc: zhangqiao22@...wei.com, Vincent Guittot <vincent.guittot@...aro.org> Subject: [PATCH v2] sched/fair: limit sched slice duration In presence of a lot of small weight tasks like sched_idle tasks, normal or high weight tasks can see their ideal runtime (sched_slice) to increase to hundreds ms whereas it normally stays below sysctl_sched_latency. 2 normal tasks running on a CPU will have a max sched_slice of 12ms (half of the sched_period). This means that they will make progress every sysctl_sched_latency period. If we now add 1000 idle tasks on the CPU, the sched_period becomes 3006 ms and the ideal runtime of the normal tasks becomes 609 ms. It will even become 1500ms if the idle tasks belongs to an idle cgroup. This means that the scheduler will look for picking another waiting task after 609ms running time (1500ms respectively). The idle tasks change significantly the way the 2 normal tasks interleave their running time slot whereas they should have a small impact. Such long sched_slice can delay significantly the release of resources as the tasks can wait hundreds of ms before the next running slot just because of idle tasks queued on the rq. Cap the ideal_runtime to the weighted version of sysctl_sched_latency when comparing with the vruntime of the next waiting task to make sure that tasks will regularly make progress and will not be significantly impacted by idle/background tasks queued on the rq. Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org> --- I have kept the test if (delta < 0) as calc_delta_fair() can't handle negative value. Change since v1: - the first 3 patches have been already queued - use the weight of curr to scale sysctl_sched_latency before capping the ideal_runtime so we can compare vruntime values. kernel/sched/fair.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 5ffec4370602..ba451bb25929 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4610,6 +4610,8 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr) if (delta < 0) return; + ideal_runtime = min_t(u64, ideal_runtime, + calc_delta_fair(sysctl_sched_latency, curr)); if (delta > ideal_runtime) resched_curr(rq_of(cfs_rq)); } -- 2.17.1
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