Messages in this thread Patch in this message | | | Date | Thu, 24 Apr 2014 12:00:47 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched: Skip double execution of pick_next_task_fair |
| |
On Wed, Apr 23, 2014 at 03:31:57PM -0700, Tim Chen wrote: > The current code will call pick_next_task_fair a second time > in the slow path if we did not pull any task in our first try. > This is really unnecessary as we already know no task can > be pulled and it doubles the delay for the cpu to enter idle. > > We instrumented some network workloads and that saw that > pick_next_task_fair is frequently called twice before a cpu enters idle. > The call to pick_next_task_fair can add > non trivial latency as it calls load_balance which runs find_busiest_group > on an hierarchy of sched domains spanning the cpus for a large system. For > some 4 socket systems, we saw almost 0.25 msec spent per call > of pick_next_task_fair before a cpu can be idled. > > This patch skips pick_next_task_fair in the slow path if it > has already been invoked.
How about something like so?
Its a little more contained.
--- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2636,8 +2636,14 @@ pick_next_task(struct rq *rq, struct tas if (likely(prev->sched_class == class && rq->nr_running == rq->cfs.h_nr_running)) { p = fair_sched_class.pick_next_task(rq, prev); - if (likely(p && p != RETRY_TASK)) - return p; + if (unlikely(p == RETRY_TASK)) + goto again; + + /* assumes fair_sched_class->next == idle_sched_class */ + if (unlikely(!p)) + p = pick_next_task_idle(rq, prev); + + return p; } again:
| |