linux-kernel - Re: [RFC][PATCH 1/3] sched: Rewrite tg_shares

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Message-ID: <20100830172031.GA17281@linux.vnet.ibm.com>
Date:	Mon, 30 Aug 2010 22:50:31 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Paul Turner <pjt@...gle.com>,
	Chris Friesen <cfriesen@...tel.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Pierre Bourdon <pbourdon@...ellency.fr>
Subject: Re: [RFC][PATCH 1/3] sched: Rewrite tg_shares_up

On Sun, Aug 29, 2010 at 12:30:26AM +0200, Peter Zijlstra wrote:
> By tracking a per-cpu load-avg for each cfs_rq and folding it into a
> global task_group load on each tick we can rework tg_shares_up to be
> strictly per-cpu.

So tg->load_weight is supposed to represent more or less current task load
across all cpus? I see only atomic_add() to it - which means it can only keep
growing or remain constant - IOW capturing the historical load even since
the task group was started? I was expecting it to reduce based on how a group 
goes idle, otherwise 

> +static void update_cfs_shares(struct cfs_rq *cfs_rq)
> +{
> +	struct task_group *tg;
> +	struct sched_entity *se;
> +	unsigned long load_weight, load, shares;
> +
> +	if (!cfs_rq)
> +		return;
> +
> +	tg = cfs_rq->tg;
> +	se = tg->se[cpu_of(rq_of(cfs_rq))];
> +	if (!se)
> +		return;
> +
> +	load = cfs_rq->load.weight;
> +
> +	load_weight = atomic_read(&tg->load_weight);
> +	load_weight -= cfs_rq->load_contribution;
> +	load_weight += load;
> +
> +	shares = (tg->shares * load);
> +	if (load_weight)
> +		shares /= load_weight;

this seem incorrect? Even though we have corrected tg->load_weight to reflect
current load on 'cfs_rq', tg->load_weight still captures historical load on
other cpus and hence could be a large #, making the division inaccurate?

Also I wonder how much of a hot spot tg->load_weight would become ..

- vatsa
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