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

Vincent Guittot
2015-08-11T16:37:36Z

On 11 August 2015 at 17:07, Juri Lelli <juri.lelli@arm.com> wrote:
> Hi Vincent,
>
> On 11/08/15 12:41, Vincent Guittot wrote:
>> On 11 August 2015 at 11:08, Juri Lelli <juri.lelli@arm.com> wrote:
>>> On 10/08/15 16:07, Vincent Guittot wrote:
>>>> 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
>>>>
>>>
>>> The tick is used to decide if we need to scale up (to max OPP for the
>>> time being), but we don't scale down. It makes more logical sense to
>>
>> why don't you want to check if you need to scale down ?
>>
>
> Well, because if I'm still executing something the cpu usage is only
> subject to raise.

This is only true for system with NO HZ idle

>
>>> scale down at task deactivation, or wakeup after a long time, IMHO.
>>
>> But waking up or going to sleep don't have any impact on the usage of
>> a cpu. The only events that impact the cpu usage are:
>> -task migration,
>
> We explicitly cover this on load balancing paths.
>
>> -new task
>
> We cover this in enqueue_task_fair(), introducing a new flag.
>
>> -time that elapse which can be monitored by periodically checking the usage.
>
> Do you mean when a task utilization crosses some threshold
> related to the current OPP? If that is the case, we have a
> check in task_tick_fair().
>
>> -and for nohz system when cpu enter or leave idle state
>>
>
> We address this in dequeue_task_fair(). In particular, if
> the cpu is going to be idle we don't trigger any change as
> it seems not always wise to wake up a thread to just change
> the OPP and the go idle; some platforms might require this
> behaviour anyway, but it probably more cpuidle/fw related?

I would say that it's interesting to notifiy sched-dvfs that a cpu
becomes idle because we could decrease the opp of a cluster of cpus
that share the same clock if this cpu is the one that requires the max
capacity of the cluster (and other cpus are still running).

>
> I would also add:
>
> - task is going to die
>
> We address this in dequeue as well, as its contribution is
> removed from usage (mod Yuyang's patches).
>
>> waking up and going to sleep events doesn't give any useful
>> information and using them to trig the monitoring of the usage
>> variation doesn't give you a predictable/periodic update of it whereas
>> the tick will
>>
>
> So, one key point of this solution is to get away as much
> as we can from periodic updates/sampling and move towards a
> (fully) event driven approach. The event logically associated
> to task_tick_fair() is when we realize that a task is going
> to saturate the current capacity; in this case we trigger a
> freq switch to an higher capacity. Also, if we never react
> to normal wakeups (as I understand you are proposing) we might
> miss some chances to adapt quickly enough. As an example, if
> you have a big task that suddenly goes to sleep, and sleeps
> until its decayed utilization goes almost to zero; when it
> wakes up, if we don't have a trigger in enqueue_task_fair(),
> we'll have to wait until the next tick to select an appropriate
> (low) OPP.

I assume that the cpu is idle in this case. This situation only
happens on Nohz idle system because tick is disable and you have to
update statistics when leaving idle as it is done for the jiffies or
the cpu_load array. So you should track cpu enter/leave idle (for nohz
system only) instead of tracking all tasks wake up/sleep events.

So you can either use update_cpu_load_nohz like it is already done for
cpu_load array
or you should use some conditions like below if you want to stay in
enqueue/dequeue_task_fair but task wake up or sleep event are not the
right condition
if (!(flags & ENQUEUE_WAKEUP) || rq->nr_running == 1 ) in enqueue_task_fair
and
if (!task_sleep || rq->nr_running == 0) in dequeue_task_fair

We can probably optimized by using rq->cfs.h_nr_running instead of
rq->nr_running as only cfs tasks really modifies the usage

Regards,
Vincent

>
> Best,
>
> - Juri
>
>>>
>>> Best,
>>>
>>> - Juri
>>>
>>>> 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
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>