Messages in this thread Patch in this message | | | Date | Fri, 21 Nov 2014 12:35:59 +0000 | From | Morten Rasmussen <> | Subject | Re: [PATCH v9 05/10] sched: make scale_rt invariant with frequency |
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On Mon, Nov 03, 2014 at 04:54:42PM +0000, 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 > > 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 ?
It better be fast :) It is used in critical paths. However, if you really care about latency you probably don't want frequency scaling to mess around. If the architecture provides a fast-path for arch_scale_freq_capacity() returning SCHED_CAPACITY_SCALE when frequency scaling is disabled, the overhead should be minimal. If the architecture doesn't provide arch_scale_freq_capacity() it becomes a constant multiplication and should hopefully go away completely.
> > 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 > @@ -5785,7 +5785,7 @@ unsigned long __weak arch_scale_cpu_capacity(struct sched_domain *sd, int cpu) > static unsigned long scale_rt_capacity(int cpu) > { > struct rq *rq = cpu_rq(cpu); > - u64 total, available, age_stamp, avg; > + u64 total, used, age_stamp, avg; > s64 delta; > > /* > @@ -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);
I haven't looked through all the details of the rt avg tracking, but if 'used' is in the range [0..SCHED_CAPACITY_SCALE], I believe it should work. Is it guaranteed that total > 0 so we don't get division by zero?
It does get a slightly more complicated if we want to figure out the available capacity at the current frequency (current < max) later. Say, rt eats 25% of the compute capacity, but the current frequency is only 50%. In that case get:
curr_avail_capacity = (arch_scale_cpu_capacity() * (arch_scale_freq_capacity() - (SCHED_SCALE_CAPACITY - scale_rt_capacity()))) >> SCHED_CAPACITY_SHIFT
With numbers assuming arch_scale_cpu_capacity() = 800:
curr_avail_capacity = 800 * (512 - (1024 - 758)) >> 10 = 200
Which isn't actually that bad. Anyway, it isn't needed until we start invovling energy models.
> > - 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; > } > > static void update_cpu_capacity(struct sched_domain *sd, int cpu) > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > index c34bd11..fc5b152 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -1312,9 +1312,11 @@ static inline int hrtick_enabled(struct rq *rq) > > #ifdef CONFIG_SMP > extern void sched_avg_update(struct rq *rq); > +extern unsigned long arch_scale_freq_capacity(struct sched_domain *sd, int cpu);
I'm not sure if it makes any difference, but shouldn't it be __weak instead of extern?
unsigned long __weak arch_scale_freq_capacity(...)
Also, now that the function prototype definition is in the header file we can kill the local prototype in fair.c introduced in patch 4:
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 6fd5ac6..921b174 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -2277,8 +2277,6 @@ static u32 __compute_runnable_contrib(u64 n) return contrib + runnable_avg_yN_sum[n]; } -unsigned long __weak arch_scale_freq_capacity(struct sched_domain *sd, int cpu); - /* * We can represent the historical contribution to runnable average as * the * coefficients of a geometric series. To do this we sub-divide our * runnable
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