linux-kernel - Re: [PATCH v1 00/10] Optimize sched avgs computation and implement flat util hierarchy

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Message-ID: <474c486c-84b3-14b0-6861-336a129f7ed8@arm.com>
Date:   Tue, 23 Aug 2016 16:39:51 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Yuyang Du <yuyang.du@...el.com>, Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Benjamin Segall <bsegall@...gle.com>,
        Paul Turner <pjt@...gle.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Mike Galbraith <umgwanakikbuti@...il.com>
Subject: Re: [PATCH v1 00/10] Optimize sched avgs computation and implement
 flat util hierarchy

On 23/08/16 15:45, Vincent Guittot wrote:
> On 23 August 2016 at 16:13, Peter Zijlstra <peterz@...radead.org> wrote:
>> On Tue, Aug 23, 2016 at 03:28:19PM +0200, Vincent Guittot wrote:
>>> I still wonder if using a flat util hierarchy is the right solution to
>>> solve this problem with utilization and task group. I have noticed
>>> exact same issues with load that generates weird task placement
>>> decision and i think that we should probably try to solve both wrong
>>> behavior with same mechanism. but this is not possible with flat
>>> hierarchy for load
>>>
>>> Let me take an example.
>>> TA is a always running task on CPU1 in group /root/level1/
>>> TB wakes up on CPU0 and moves TA into group /root/level2/
>>> Even if TA stays on CPU1, runnable_load_avg of CPU1 root cfs rq will become 0.
>>
>> Because while we migrate the load_avg on /root/level2, we do not
>> propagate the load_avg up the hierarchy?
> 
> yes. At now, the load of a cfs_rq and the load of its sched_entity
> that represents it at parent level are disconnected

I guess you say 'disconnected' because cfs_rq and se (w/ cfs_rq eq.
se->my_q) are now independent pelt signals where as before the rewrite
they were 'connected' for load via __update_tg_runnable_avg(),
__update_group_entity_contrib() in __update_entity_load_avg_contrib()
and for utilization via 'se->avg.utilization_avg_contrib =
group_cfs_rq(se)->utilization_load_avg' in
__update_entity_utilization_avg_contrib().

IMHO, there was also this 'connection' between se and cfs_rq (w/
se->cfs_rq eq. cfs_rq) in update_entity_load_avg(, update_cfs_rq = 1)
which guaranteed that the change in the se was immediately visible on
the cfs_rq representing the parent task group.

 if (se->on_rq) {
   cfs_rq->runnable_load_avg += contrib_delta;
   cfs_rq->utilization_load_avg += utilization_delta;
 }

I guess, these two things somehow belonged together to achieve this
load/util propagation.

>> And always propagating everyrthing up will indeed also fix the
>> utilization issue.
>>
>> Of course, doing that propagation has its costs..
> 
> yes, that's the counterpart
> 
>>
>> Didn't you post a patch doing just this a while ago?
> 
> My patch was doing that but only for utilization and i have start to
> work on adding the propagation of load as well
>