linux-kernel - Re: [PATCH 5/7 v4] sched: propagate asynchrous detach

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Date:   Wed, 12 Oct 2016 17:45:59 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Wanpeng Li <kernellwp@...il.com>,
        Yuyang Du <yuyang.du@...el.com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
        Paul Turner <pjt@...gle.com>,
        Benjamin Segall <bsegall@...gle.com>
Subject: Re: [PATCH 5/7 v4] sched: propagate asynchrous detach

On 12 October 2016 at 17:03, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> On 26/09/16 13:19, Vincent Guittot wrote:
>> A task can be asynchronously detached from cfs_rq when migrating
>> between CPUs. The load of the migrated task is then removed from
>> source cfs_rq during its next update. We use this event to set propagation
>> flag.
>>
>> During the load balance, we take advanatge of the update of blocked load
>> to we propagate any pending changes.
>
> IMHO, it would be a good idea to mention that '2/7 sched: fix
> hierarchical order in rq->leaf_cfs_rq_list' is a hard requirement for
> this to work. The functionality relies on the order of cfs_rq's (top to
> root) in the rq->leaf_cfs_rq_list list.

yes. i will add a comment

>
>> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
>> ---
>>  kernel/sched/fair.c | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 8ba500f..bd3b6b9 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -3221,6 +3221,7 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq, bool update_freq)
>>               sub_positive(&sa->load_avg, r);
>>               sub_positive(&sa->load_sum, r * LOAD_AVG_MAX);
>>               removed_load = 1;
>> +             set_tg_cfs_propagate(cfs_rq);
>>       }
>>
>>       if (atomic_long_read(&cfs_rq->removed_util_avg)) {
>> @@ -3228,6 +3229,7 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq, bool update_freq)
>>               sub_positive(&sa->util_avg, r);
>>               sub_positive(&sa->util_sum, r * LOAD_AVG_MAX);
>>               removed_util = 1;
>> +             set_tg_cfs_propagate(cfs_rq);
>>       }
>>
>>       decayed = __update_load_avg(now, cpu_of(rq_of(cfs_rq)), sa,
>> @@ -6607,6 +6609,10 @@ static void update_blocked_averages(int cpu)
>>
>>               if (update_cfs_rq_load_avg(cfs_rq_clock_task(cfs_rq), cfs_rq, true))
>>                       update_tg_load_avg(cfs_rq, 0);
>> +
>> +             /* Propagate pending load changes to the parent */
>> +             if (cfs_rq->tg->se[cpu])
>> +                     update_load_avg(cfs_rq->tg->se[cpu], 0);
>
> In my test (1 task (run/period: 8ms/16ms) in tg_root->tg_x->tg_y->*tg_z*
> and oscillating between cpu1 and cpu2) the cfs_rq related signals are
> nicely going down to 0 after the task has left the cpu but it doesn't
> seem to be the case for the corresponding se (cfs_rq->tg->se[cpu])?

strange because such use case is part of the functional tests that I
run and it was working fine according to last test that I did

>
> It should actually work correctly because of the
> update_tg_cfs_util/load() calls in update_load_avg(cfs_rq->tg->se[cpu],
> 0)->propagate_entity_load_avg()

Furthermore, the update of the parent cfs_rq tg_x->cfs_rq[cpu] uses
the delta between previous and new value for the child tg_y->se[cpu].
So it means that if tg_x->cfs_rq[cpu]->avg.load_avg goes down to 0,
tg_y->se[cpu]->avg.load_avg has at least changed and most probably
goes down to 0 too

Can't it be a misplaced trace point ?

>
> [...]