linux-kernel - Re: [PATCH] sched: make update_sd_pick

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Message-ID: <53D26383.60707@redhat.com>
Date:	Fri, 25 Jul 2014 10:02:43 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
CC:	linux-kernel <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Michael Neuling <mikey@...ling.org>,
	Ingo Molnar <mingo@...nel.org>, Paul Turner <pjt@...gle.com>,
	jhladky@...hat.com, ktkhai@...allels.com,
	tim.c.chen@...ux.intel.com,
	Nicolas Pitre <nicolas.pitre@...aro.org>
Subject: Re: [PATCH] sched: make update_sd_pick_busiest return true on a busier
 sd

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On 07/23/2014 03:41 AM, Vincent Guittot wrote:

> Regarding your issue with "perf bench numa mem" that is not spread
> on all nodes, SD_PREFER_SIBLING flag (of DIE level) should do the
> job by reducing the capacity of  "not local DIE" group at NUMA
> level to 1 task during the load balance computation. So you should
> have 1 task per sched_group at NUMA level.

Looking at the code some more, it is clear why this does not
happen. If sd->flags & SD_NUMA, then SD_PREFER_SIBLING will
never be set.

On a related note, that part of the load balancing code probably
needs to be rewritten to deal with unequal group_capacity_factors
anyway.

Say that one group has a group_capacity_factor twice that of
another group.

The group with the smaller group_capacity_factor is overloaded
by a factor 1.3. The larger group is loaded by a factor 0.8.
This means the larger group has a higher load than the first
group, and the current code in update_sd_pick_busiest will
not select the overloaded group as the busiest one, due to not
scaling load with the capacity...

static bool update_sd_pick_busiest(struct lb_env *env,
                                   struct sd_lb_stats *sds,
                                   struct sched_group *sg,
                                   struct sg_lb_stats *sgs)
{
        if (sgs->avg_load <= sds->busiest_stat.avg_load)
                return false;

I believe we may need to factor the group_capacity_factor
into this calculation, in order to properly identify which
group is busiest.

However, if we do that we may need to get rid of the
SD_PREFER_SIBLING hack that forces group_capacity_factor
to 1 on domains that have SD_PREFER_SIBLING set.

I suspect that should be ok though, if we make sure
update_sd_pick_busiest does the right thing...

- -- 
All rights reversed
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