linux-kernel - Re: [PATCH 4/7 v3] sched: propagate load during synchronous attach/detach

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Message-ID: <038286f2-02bf-e635-e60a-33904e38826c@arm.com>
Date:   Thu, 15 Sep 2016 18:36:53 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>, mingo@...nel.org,
        linux-kernel@...r.kernel.org, yuyang.du@...el.com,
        Morten.Rasmussen@....com, linaro-kernel@...ts.linaro.org,
        pjt@...gle.com, bsegall@...gle.com
Subject: Re: [PATCH 4/7 v3] sched: propagate load during synchronous
 attach/detach

On 15/09/16 16:14, Peter Zijlstra wrote:
> On Thu, Sep 15, 2016 at 02:11:49PM +0100, Dietmar Eggemann wrote:
>> On 12/09/16 08:47, Vincent Guittot wrote:
> 
>>> +/* Take into account change of load of a child task group */
>>> +static inline void
>>> +update_tg_cfs_load(struct cfs_rq *cfs_rq, struct sched_entity *se)
>>> +{
>>> +	struct cfs_rq *gcfs_rq = group_cfs_rq(se);
>>> +	long delta, load = gcfs_rq->avg.load_avg;
>>> +
>>> +	/* If the load of group cfs_rq is null, the load of the
>>> +	 * sched_entity will also be null so we can skip the formula
>>> +	 */
>>> +	if (load) {
>>> +		long tg_load;
>>> +
>>> +		/* Get tg's load and ensure tg_load > 0 */
>>> +		tg_load = atomic_long_read(&gcfs_rq->tg->load_avg) + 1;
>>> +
>>> +		/* Ensure tg_load >= load and updated with current load*/
>>> +		tg_load -= gcfs_rq->tg_load_avg_contrib;
>>> +		tg_load += load;
>>> +
>>> +		/* scale gcfs_rq's load into tg's shares*/
>>> +		load *= scale_load_down(gcfs_rq->tg->shares);
>>> +		load /= tg_load;
>>> +
>>> +		/*
>>> +		 * we need to compute a correction term in the case that the
>>> +		 * task group is consuming <1 cpu so that we would contribute
>>> +		 * the same load as a task of equal weight.
>>
>> Wasn't 'consuming <1' related to 'NICE_0_LOAD' and not
>> scale_load_down(gcfs_rq->tg->shares) before the rewrite of PELT (v4.2,
>> __update_group_entity_contrib())?
> 
> 
> So the approximation was: min(1, runnable_avg) * shares;
> 
> And it just so happened that we tracked runnable_avg in 10 bit fixed
> point, which then happened to be NICE_0_LOAD.
> 
> But here we have load_avg, which already includes a '* shares' factor.
> So that then becomes min(shares, load_avg).

Makes sense, understand it now.

> We did however loose a lot on why and how min(1, runnable_avg) is a
> sensible thing to do...

Do you refer to the big comment on top of this if condition in the old
code in __update_group_entity_contrib()? The last two subsections of it
I never understood ...

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