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sched/core: Rework rq->clock update skips

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The original purpose of rq::skip_clock_update was to avoid 'costly' clock
updates for back to back wakeup-preempt pairs. The big problem with it
has always been that the rq variable is unaware of the context and
causes indiscrimiate clock skips.

Rework the entire thing and create a sense of context by only allowing
schedule() to skip clock updates. (XXX can we measure the cost of the
added store?)

By ensuring only schedule can ever skip an update, we guarantee we're
never more than 1 tick behind on the update.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: umgwanakikbuti@gmail.com
Link: http://lkml.kernel.org/r/20150105103554.432381549@infradead.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra
2015-01-05 11:18:11 +01:00
committed by Ingo Molnar
parent cebde6d681
commit 9edfbfed3f
4 changed files with 28 additions and 10 deletions
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9
kernel/sched/rt.c
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@@ -831,11 +831,14 @@ static int do_sched_rt_period_timer(struct rt_bandwidth *rt_b, int overrun)
enqueue = 1;
/*
* Force a clock update if the CPU was idle,
* lest wakeup -> unthrottle time accumulate.
* When we're idle and a woken (rt) task is
* throttled check_preempt_curr() will set
* skip_update and the time between the wakeup
* and this unthrottle will get accounted as
* 'runtime'.
*/
if (rt_rq->rt_nr_running && rq->curr == rq->idle)
rq->skip_clock_update = -1;
rq_clock_skip_update(rq, false);
}
if (rt_rq->rt_time || rt_rq->rt_nr_running)
idle = 0;