From f6cf891c4d7128f9f91243fc0b9ce99e10fa1586 Mon Sep 17 00:00:00 2001 From: Ingo Molnar Date: Tue, 28 Aug 2007 12:53:24 +0200 Subject: sched: make the scheduler converge to the ideal latency de-HZ-ification of the granularity defaults unearthed a pre-existing property of CFS: while it correctly converges to the granularity goal, it does not prevent run-time fluctuations in the range of [-gran ... 0 ... +gran]. With the increase of the granularity due to the removal of HZ dependencies, this becomes visible in chew-max output (with 5 tasks running): out: 28 . 27. 32 | flu: 0 . 0 | ran: 9 . 13 | per: 37 . 40 out: 27 . 27. 32 | flu: 0 . 0 | ran: 17 . 13 | per: 44 . 40 out: 27 . 27. 32 | flu: 0 . 0 | ran: 9 . 13 | per: 36 . 40 out: 29 . 27. 32 | flu: 2 . 0 | ran: 17 . 13 | per: 46 . 40 out: 28 . 27. 32 | flu: 0 . 0 | ran: 9 . 13 | per: 37 . 40 out: 29 . 27. 32 | flu: 0 . 0 | ran: 18 . 13 | per: 47 . 40 out: 28 . 27. 32 | flu: 0 . 0 | ran: 9 . 13 | per: 37 . 40 average slice is the ideal 13 msecs and the period is picture-perfect 40 msecs. But the 'ran' field fluctuates around 13.33 msecs and there's no mechanism in CFS to keep that from happening: it's a perfectly valid solution that CFS finds. to fix this we add a granularity/preemption rule that knows about the "target latency", which makes tasks that run longer than the ideal latency run a bit less. The simplest approach is to simply decrease the preemption granularity when a task overruns its ideal latency. For this we have to track how much the task executed since its last preemption. ( this adds a new field to task_struct, but we can eliminate that overhead in 2.6.24 by putting all the scheduler timestamps into an anonymous union. ) with this change in place, chew-max output is fluctuation-less all around: out: 28 . 27. 39 | flu: 0 . 2 | ran: 13 . 13 | per: 41 . 40 out: 28 . 27. 39 | flu: 0 . 2 | ran: 13 . 13 | per: 41 . 40 out: 28 . 27. 39 | flu: 0 . 2 | ran: 13 . 13 | per: 41 . 40 out: 28 . 27. 39 | flu: 0 . 2 | ran: 13 . 13 | per: 41 . 40 out: 28 . 27. 39 | flu: 0 . 1 | ran: 13 . 13 | per: 41 . 40 out: 28 . 27. 39 | flu: 0 . 1 | ran: 13 . 13 | per: 41 . 40 this patch has no impact on any fastpath or on any globally observable scheduling property. (unless you have sharp enough eyes to see millisecond-level ruckles in glxgears smoothness :-) Signed-off-by: Ingo Molnar Signed-off-by: Peter Zijlstra Signed-off-by: Mike Galbraith --- kernel/sched_fair.c | 26 ++++++++++++++++++++++---- 1 file changed, 22 insertions(+), 4 deletions(-) (limited to 'kernel/sched_fair.c') diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c index 9f53d49f3aa..721fe774487 100644 --- a/kernel/sched_fair.c +++ b/kernel/sched_fair.c @@ -668,7 +668,7 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int sleep) /* * Preempt the current task with a newly woken task if needed: */ -static void +static int __check_preempt_curr_fair(struct cfs_rq *cfs_rq, struct sched_entity *se, struct sched_entity *curr, unsigned long granularity) { @@ -679,8 +679,11 @@ __check_preempt_curr_fair(struct cfs_rq *cfs_rq, struct sched_entity *se, * preempt the current task unless the best task has * a larger than sched_granularity fairness advantage: */ - if (__delta > niced_granularity(curr, granularity)) + if (__delta > niced_granularity(curr, granularity)) { resched_task(rq_of(cfs_rq)->curr); + return 1; + } + return 0; } static inline void @@ -725,6 +728,7 @@ static void put_prev_entity(struct cfs_rq *cfs_rq, struct sched_entity *prev) static void entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr) { + unsigned long gran, ideal_runtime, delta_exec; struct sched_entity *next; /* @@ -741,8 +745,22 @@ static void entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr) if (next == curr) return; - __check_preempt_curr_fair(cfs_rq, next, curr, - sched_granularity(cfs_rq)); + gran = sched_granularity(cfs_rq); + ideal_runtime = niced_granularity(curr, + max(sysctl_sched_latency / cfs_rq->nr_running, + (unsigned long)sysctl_sched_min_granularity)); + /* + * If we executed more than what the latency constraint suggests, + * reduce the rescheduling granularity. This way the total latency + * of how much a task is not scheduled converges to + * sysctl_sched_latency: + */ + delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime; + if (delta_exec > ideal_runtime) + gran = 0; + + if (__check_preempt_curr_fair(cfs_rq, next, curr, gran)) + curr->prev_sum_exec_runtime = curr->sum_exec_runtime; } /************************************************** -- cgit v1.2.3