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author | Ingo Molnar <mingo@elte.hu> | 2007-08-28 12:53:24 +0200 |
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committer | Ingo Molnar <mingo@elte.hu> | 2007-08-28 12:53:24 +0200 |
commit | f6cf891c4d7128f9f91243fc0b9ce99e10fa1586 (patch) | |
tree | ab26a8e708756c3fbafbb15ce48acea4f80ced08 /kernel/sched_fair.c | |
parent | 5f01d519e60a6ca1a7d9be9f2d73c5f521383992 (diff) | |
download | op-kernel-dev-f6cf891c4d7128f9f91243fc0b9ce99e10fa1586.zip op-kernel-dev-f6cf891c4d7128f9f91243fc0b9ce99e10fa1586.tar.gz |
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 <mingo@elte.hu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Diffstat (limited to 'kernel/sched_fair.c')
-rw-r--r-- | kernel/sched_fair.c | 26 |
1 files changed, 22 insertions, 4 deletions
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c index 9f53d49..721fe77 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; } /************************************************** |