linux-kernel - Re: [RFC PATCH 0/1] sched/pelt: Change PELT halflife at runtime

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Message-ID: <2eb769e6-0749-0cd2-3c1c-44262b3a8110@arm.com>
Date:   Wed, 30 Nov 2022 19:14:51 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Kajetan Puchalski <kajetan.puchalski@....com>,
        Jian-Min Liu <jian-min.liu@...iatek.com>,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Vincent Donnefort <vdonnefort@...gle.com>,
        Quentin Perret <qperret@...gle.com>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Abhijeet Dharmapurikar <adharmap@...cinc.com>,
        Qais Yousef <qais.yousef@....com>,
        linux-kernel@...r.kernel.org,
        Jonathan JMChen <jonathan.jmchen@...iatek.com>
Subject: Re: [RFC PATCH 0/1] sched/pelt: Change PELT halflife at runtime

On 10/11/2022 18:51, Peter Zijlstra wrote:
> On Thu, Nov 10, 2022 at 03:59:01PM +0100, Dietmar Eggemann wrote:
>> On 10/11/2022 14:05, Peter Zijlstra wrote:
>>> On Thu, Nov 10, 2022 at 12:16:26PM +0100, Dietmar Eggemann wrote:
>>>> On 07/11/2022 14:41, Peter Zijlstra wrote:
>>>>> On Thu, Sep 29, 2022 at 03:41:47PM +0100, Kajetan Puchalski wrote:
>>>>
>>>> [...]
>>>>
>>>>> @@ -2956,13 +2958,26 @@ static inline unsigned long cpu_util_dl(struct rq *rq)
>>>>>   */
>>>>>  static inline unsigned long cpu_util_cfs(int cpu)
>>>>>  {
>>>>> +	struct rq *rq = cpu_rq(cpu);
>>>>>  	struct cfs_rq *cfs_rq;
>>>>>  	unsigned long util;
>>>>>  
>>>>> -	cfs_rq = &cpu_rq(cpu)->cfs;
>>>>> +	cfs_rq = &rq->cfs;
>>>>>  	util = READ_ONCE(cfs_rq->avg.util_avg);
>>>>>  
>>>>>  	if (sched_feat(UTIL_EST)) {
>>>>> +		if (sched_feat(UTIL_EST_FASTER)) {
>>>>> +			struct task_struct *curr;
>>>>> +
>>>>> +			rcu_read_lock();
>>>>> +			curr = rcu_dereference(rq->curr);
>>>>> +			if (likely(curr->sched_class == &fair_sched_class)) {
>>>>> +				u64 runtime = curr->se.sum_exec_runtime - curr->se.exec_start;
>>>>
>>>> Don't we and up with gigantic runtime numbers here?
>>>>
>>>> oot@...o:~# cat /proc/1676/task/1676/schedstat
>>>> 36946300 1150620 11
>>>> root@...o:~# cat /proc/1676/task/1676/sched
>>>> rt-app (1676, #threads: 2)
>>>> -------------------------------------------------------------------
>>>> se.exec_start                                :         77766.964240 <- !
>>>> se.vruntime                                  :           563.587883
>>>> e.sum_exec_runtime                          :            36.946300  <- !
>>>> se.nr_migrations                             :                    0
>>>> ...
>>>>
>>>> I expect cpu_util_cfs() to be ~1024 almost all the time now.
>>>
>>> Duh, obviously I meant to measure the runtime of the current activation
>>> and messed up.
>>>
>>> We don't appear to have the right information to compute this atm :/
>>
>> This would be:
>>
>> u64 now = rq_clock_task(rq);
>> u64 runtime = now - curr->se.exec_start;
>>
>> but we don't hold the rq lock so we can't get `now`?
> 
> Not quite the same; that's the time since we got on-cpu last, but that's
> not the same as the runtime of this activation (it is when you discount
> preemption).


----|----|----|----|----|----|--->
    a    s1   p1   s2   p2   d

a ... activate_task() -> enqueue_task()

s ... set_next_entity()

p ... put_prev_entity()

d ... deactivate_task() -> dequeue_task()

By `runtime of the activation` you refer to `curr->sum_exec_runtime -
time(a)` ? And the latter we don't have?

And `runtime = curr->se.sum_exec_runtime - curr->se.prev_sum_exec_run`
is only covering the time since we got onto the cpu, right?

With a missing `runtime >>= 10` (from __update_load_sum()) and using
`runtime = curr->se.sum_exec_runtime - curr->se.prev_sum_exec_runtime`
for a 1 task-workload (so no preemption) with factor 2 or 4 I get at
least close to the original rq->cfs.avg.util_avg and util_est.enqueued
signals (cells (5)-(8) in the notebook below).

https://nbviewer.org/github/deggeman/lisa/blob/ipynbs/ipynb/scratchpad/UTIL_EST_FASTER.ipynb?flush_cache=true

----

set_next_entity()
    update_stats_curr_start()
        se->exec_start = rq_clock_task()

    cfs_rq->curr = se                                (1)

    se->prev_sum_exec_runtime = se->sum_exec_runtime (2)

update_curr()

    now = rq_clock_task(rq_of(cfs_rq))
    delta_exec = now - curr->exec_start              (3)
    curr->exec_start = now
    curr->sum_exec_runtime += delta_exec;            (4)

put_prev_entity()

    cfs_rq->curr = NULL                              (5)