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Date:   Thu, 10 Nov 2022 15:59:01 +0100
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Subject: Re: [RFC PATCH 0/1] sched/pelt: Change PELT halflife at runtime
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To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Kajetan Puchalski <kajetan.puchalski@arm.com>,
        Jian-Min Liu <jian-min.liu@mediatek.com>,
        Ingo Molnar <mingo@kernel.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Vincent Donnefort <vdonnefort@google.com>,
        Quentin Perret <qperret@google.com>,
        Patrick Bellasi <patrick.bellasi@matbug.net>,
        Abhijeet Dharmapurikar <adharmap@quicinc.com>,
        Qais Yousef <qais.yousef@arm.com>,
        linux-kernel@vger.kernel.org,
        Jonathan JMChen <jonathan.jmchen@mediatek.com>
References: <20220829055450.1703092-1-dietmar.eggemann@arm.com>
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From:   Dietmar Eggemann <dietmar.eggemann@arm.com>
In-Reply-To: <Y2z2/nhGfud7NIM8@hirez.programming.kicks-ass.net>
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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@juno:~# cat /proc/1676/task/1676/schedstat
>> 36946300 1150620 11
>> root@juno:~# 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`?