linux-kernel - Re: [PATCH] sched/fair: Do not decay new task load on first enqueue

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Date:   Wed, 28 Sep 2016 12:31:07 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Matt Fleming <matt@...eblueprint.co.uk>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        Mike Galbraith <umgwanakikbuti@...il.com>,
        Yuyang Du <yuyang.du@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH] sched/fair: Do not decay new task load on first enqueue

On 28/09/16 12:19, Peter Zijlstra wrote:
> On Wed, Sep 28, 2016 at 12:06:43PM +0100, Dietmar Eggemann wrote:
>> On 28/09/16 11:14, Peter Zijlstra wrote:
>>> On Fri, Sep 23, 2016 at 12:58:08PM +0100, Matt Fleming wrote:

[...]

>> Not sure what you mean by 'after fixing' but the se is initialized with
>> a possibly stale 'now' value in post_init_entity_util_avg()->
>> attach_entity_load_avg() before the clock is updated in
>> activate_task()->enqueue_task().
> 
> I meant that after I fix the above issue of calling post_init with a
> stale clock. So the + update_rq_clock(rq) in the patch.

OK.

[...]

>>> While staring at this, I don't think we can still hit
>>> vruntime_normalized() with a new task, so I _think_ we can remove that
>>> !se->sum_exec_runtime clause there (and rejoice), no?
>>
>> I'm afraid that with accurate timing we will get the same situation that
>> we add and subtract the same amount of load (probably 1024 now and not
>> 1002 (or less)) to/from cfs_rq->runnable_load_avg for the initial (fork)
>> hackbench run.
>> After all, it's 'runnable' based.
> 
> The idea was that since we now update rq clock before post_init and then
> leave it be, both post_init and enqueue see the exact same timestamp,
> and the delta is 0, resulting in no aging.
> 
> Or did I fail to make that happen?

No, but IMHO what Matt wants is ageing for the hackench tasks at the end
of their fork phase so there is a tiny amount of
cfs_rq->runnable_load_avg left on cpuX after the fork related dequeue so
the (load-based) fork-balancer chooses cpuY for the next hackbench task.
That's why he wanted to avoid the __update_load_avg(se) on enqueue (thus
adding 1024 to cfs_rq->runnable_load_avg) and do the ageing only on
dequeue (removing <1024 from cfs_rq->runnable_load_avg).