Messages in this thread | | | Date | Thu, 19 Feb 2015 17:52:41 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH RESEND v9 05/10] sched: make scale_rt invariant with frequency |
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On Thu, Jan 15, 2015 at 11:09:25AM +0100, Vincent Guittot wrote: > The average running time of RT tasks is used to estimate the remaining compute > capacity for CFS tasks. This remaining capacity is the original capacity scaled > down by a factor (aka scale_rt_capacity). This estimation of available capacity > must also be invariant with frequency scaling. > > A frequency scaling factor is applied on the running time of the RT tasks for > computing scale_rt_capacity. > > In sched_rt_avg_update, we scale the RT execution time like below: > rq->rt_avg += rt_delta * arch_scale_freq_capacity() >> SCHED_CAPACITY_SHIFT > > Then, scale_rt_capacity can be summarized by: > scale_rt_capacity = SCHED_CAPACITY_SCALE - > ((rq->rt_avg << SCHED_CAPACITY_SHIFT) / period) > > We can optimize by removing right and left shift in the computation of rq->rt_avg > and scale_rt_capacity
So far so good..
> The call to arch_scale_frequency_capacity in the rt scheduling path might be > a concern for RT folks because I'm not sure whether we can rely on > arch_scale_freq_capacity to be short and efficient ?
No, that is, arch_scale_frequency_capacity() _must_ be short and efficient, event for the fair class, its called in very hot paths.
I think we've talked about this before; this function should basically only return a cached value, which is periodically updated through some means.
But lets see, I've yet to see an actual implementation of it; and its got that sd argument, curious what you're going to do with that.
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > --- > kernel/sched/fair.c | 17 +++++------------ > kernel/sched/sched.h | 4 +++- > 2 files changed, 8 insertions(+), 13 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index a5039da..b37c27b 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c
> @@ -5801,19 +5801,12 @@ static unsigned long scale_rt_capacity(int cpu) > > total = sched_avg_period() + delta; > > - if (unlikely(total < avg)) { > - /* Ensures that capacity won't end up being negative */ > - available = 0; > - } else { > - available = total - avg; > - } > + used = div_u64(avg, total); > > - if (unlikely((s64)total < SCHED_CAPACITY_SCALE)) > - total = SCHED_CAPACITY_SCALE; > + if (likely(used < SCHED_CAPACITY_SCALE)) > + return SCHED_CAPACITY_SCALE - used; > > - total >>= SCHED_CAPACITY_SHIFT; > - > - return div_u64(available, total); > + return 1; > }
This makes a lot of changes not commented on in the Changelog.
I think it makes sense, but this has always been horrible.
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