Re: [RFCv5 PATCH 41/46] sched/fair: add triggers for OPP change requests

Vincent Guittot
2015-08-10T15:07:33Z

On 10 August 2015 at 15:43, Juri Lelli <juri.lelli@arm.com> wrote:
>
> Hi Vincent,
>
> On 04/08/15 14:41, Vincent Guittot wrote:
> > Hi Juri,
> >
> > On 7 July 2015 at 20:24, Morten Rasmussen <morten.rasmussen@arm.com> wrote:
> >> From: Juri Lelli <juri.lelli@arm.com>
> >>
> >> Each time a task is {en,de}queued we might need to adapt the current
> >> frequency to the new usage. Add triggers on {en,de}queue_task_fair() for
> >> this purpose. Only trigger a freq request if we are effectively waking up
> >> or going to sleep. Filter out load balancing related calls to reduce the
> >> number of triggers.
> >>
> >> cc: Ingo Molnar <mingo@redhat.com>
> >> cc: Peter Zijlstra <peterz@infradead.org>
> >>
> >> Signed-off-by: Juri Lelli <juri.lelli@arm.com>
> >> ---
> >> kernel/sched/fair.c | 42 ++++++++++++++++++++++++++++++++++++++++--
> >> 1 file changed, 40 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >> index f74e9d2..b8627c6 100644
> >> --- a/kernel/sched/fair.c
> >> +++ b/kernel/sched/fair.c
> >> @@ -4281,7 +4281,10 @@ static inline void hrtick_update(struct rq *rq)
> >> }
> >> #endif
> >>
> >> +static unsigned int capacity_margin = 1280; /* ~20% margin */
> >> +
> >> static bool cpu_overutilized(int cpu);
> >> +static unsigned long get_cpu_usage(int cpu);
> >> struct static_key __sched_energy_freq __read_mostly = STATIC_KEY_INIT_FALSE;
> >>
> >> /*
> >> @@ -4332,6 +4335,26 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> >> if (!task_new && !rq->rd->overutilized &&
> >> cpu_overutilized(rq->cpu))
> >> rq->rd->overutilized = true;
> >> + /*
> >> + * We want to trigger a freq switch request only for tasks that
> >> + * are waking up; this is because we get here also during
> >> + * load balancing, but in these cases it seems wise to trigger
> >> + * as single request after load balancing is done.
> >> + *
> >> + * XXX: how about fork()? Do we need a special flag/something
> >> + * to tell if we are here after a fork() (wakeup_task_new)?
> >> + *
> >> + * Also, we add a margin (same ~20% used for the tipping point)
> >> + * to our request to provide some head room if p's utilization
> >> + * further increases.
> >> + */
> >> + if (sched_energy_freq() && !task_new) {
> >> + unsigned long req_cap = get_cpu_usage(cpu_of(rq));
> >> +
> >> + req_cap = req_cap * capacity_margin
> >> + >> SCHED_CAPACITY_SHIFT;
> >> + cpufreq_sched_set_cap(cpu_of(rq), req_cap);
> >> + }
> >> }
> >> hrtick_update(rq);
> >> }
> >> @@ -4393,6 +4416,23 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> >> if (!se) {
> >> sub_nr_running(rq, 1);
> >> update_rq_runnable_avg(rq, 1);
> >> + /*
> >> + * We want to trigger a freq switch request only for tasks that
> >> + * are going to sleep; this is because we get here also during
> >> + * load balancing, but in these cases it seems wise to trigger
> >> + * as single request after load balancing is done.
> >> + *
> >> + * Also, we add a margin (same ~20% used for the tipping point)
> >> + * to our request to provide some head room if p's utilization
> >> + * further increases.
> >> + */
> >> + if (sched_energy_freq() && task_sleep) {
> >> + unsigned long req_cap = get_cpu_usage(cpu_of(rq));
> >> +
> >> + req_cap = req_cap * capacity_margin
> >> + >> SCHED_CAPACITY_SHIFT;
> >> + cpufreq_sched_set_cap(cpu_of(rq), req_cap);
> >
> > Could you clarify why you want to trig a freq switch for tasks that
> > are going to sleep ?
> > The cpu_usage should not changed that much as the se_utilization of
> > the entity moves from utilization_load_avg to utilization_blocked_avg
> > of the rq and the usage and the freq are updated periodically.
>
> I think we still need to cover multiple back-to-back dequeues. Suppose
> that you have, let's say, 3 tasks that get enqueued at the same time.
> After some time the first one goes to sleep and its utilization, as you
> say, gets moved to utilization_blocked_avg. So, nothing changes, and
> the trigger is superfluous (even if no freq change I guess will be
> issued as we are already servicing enough capacity). However, after a
> while, the second task goes to sleep. Now we still use get_cpu_usage()
> and the first task contribution in utilization_blocked_avg should have
> been decayed by this time. Same thing may than happen for the third task
> as well. So, if we don't check if we need to scale down in
> dequeue_task_fair, it seems to me that we might miss some opportunities,
> as blocked contribution of other tasks could have been successively
> decayed.
>
> What you think?

The tick is used to monitor such variation of the usage (in both way,
decay of the usage of sleeping tasks and increase of the usage of
running tasks). So in your example, if the duration between the sleep
of the 2 tasks is significant enough, the tick will handle this
variation

Regards,
Vincent
>
> Thanks,
>
> - Juri
>
> > It should be the same for the wake up of a task in enqueue_task_fair
> > above, even if it's less obvious for this latter use case because the
> > cpu might wake up from a long idle phase during which its
> > utilization_blocked_avg has not been updated. Nevertheless, a trig of
> > the freq switch at wake up of the cpu once its usage has been updated
> > should do the job.
> >
> > So tick, migration of tasks, new tasks, entering/leaving idle state of
> > cpu should be enough to trig freq switch
> >
> > Regards,
> > Vincent
> >
> >
> >> + }
> >> }
> >> hrtick_update(rq);
> >> }
> >> @@ -4959,8 +4999,6 @@ static int find_new_capacity(struct energy_env *eenv,
> >> return idx;
> >> }
> >>
> >> -static unsigned int capacity_margin = 1280; /* ~20% margin */
> >> -
> >> static bool cpu_overutilized(int cpu)
> >> {
> >> return (capacity_of(cpu) * 1024) <
> >> --
> >> 1.9.1
> >>
> >
>