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

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Date:	Fri, 25 Jul 2014 18:05:49 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, mikey@...ling.org, mingo@...nel.org,
	pjt@...gle.com, jhladky@...hat.com, ktkhai@...allels.com,
	tim.c.chen@...ux.intel.com, nicolas.pitre@...aro.org
Subject: Re: [PATCH] sched: make update_sd_pick_busiest return true on a
 busier sd

On Fri, Jul 25, 2014 at 11:45:02AM -0400, Rik van Riel wrote:
> > So for !ASYM the code is effectively:
> > 
> > return sgs->avg_load > sds->busiest_stat.avg_load;
> > 
> > I'd like to at least add a clause that makes overloaded groups 
> > prioritized over !overloaded groups.
> > 
> > Also, like we found earlier, calculate_imbalance() relies on the 
> > sum_nr_running > group_capacity_factor thing, which you've just 
> > 'wrecked', so we'd need an update to that part too.
> 
> I guess that would mean update_sd_pick_busiest would look like
> this for the !ASYM case:
> 
> 1) remembering whether busiest is overloaded
> 
> 2) if (sgs->busiest_stat.overloaded && !sgs->overloaded)
>             return false;
> 
> 3) if (sgs->avg_load > sds->busiest_stat.avg_load)
>             return true;

Right. So the situation I was worried about is where we have say 8 +nice
tasks in one group and 2 -nice in another. So the group with 2 tasks can
actually be 'heavier' but not fully utilized. In that case we want to
pull tasks from the lighter but overloaded group.

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