linux-kernel - Re: [PATCH v3 09/12] sched/numa: fix task scan rate adjustment

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Date:	Tue, 10 Dec 2013 08:27:59 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>, Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3 09/12] sched/numa: fix task scan rate adjustment

On Tue, Dec 10, 2013 at 01:44:35AM -0500, Naoya Horiguchi wrote:
> Hi Wanpeng,
> 
> On Sun, Dec 08, 2013 at 02:14:50PM +0800, Wanpeng Li wrote:
> > commit 04bb2f947 (sched/numa: Adjust scan rate in task_numa_placement) calculate
> > period_slot which should be used as base value of scan rate increase if remote
> > access dominate. However, current codes forget to use it, this patch fix it.
> > 
> > Signed-off-by: Wanpeng Li <liwanp@...ux.vnet.ibm.com>
> > ---
> >  kernel/sched/fair.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 7073c76..b077f1b3 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -1358,7 +1358,7 @@ static void update_task_scan_period(struct task_struct *p,
> >  		 */
> >  		period_slot = DIV_ROUND_UP(diff, NUMA_PERIOD_SLOTS);
> >  		ratio = DIV_ROUND_UP(private * NUMA_PERIOD_SLOTS, (private + shared));
> > -		diff = (diff * ratio) / NUMA_PERIOD_SLOTS;
> > +		diff = (period_slot * ratio) / NUMA_PERIOD_SLOTS;
> >  	}
> >  
> >  	p->numa_scan_period = clamp(p->numa_scan_period + diff,
> 
> It seems to me that the original code is correct, because the mathematical
> meaning of this hunk is clear:
> 
>   diff = (diff calculated by local-remote ratio) * (private-shared ratio)
> 

Thanks Naoya.

The original code is as intended and was meant to scale the difference
between the NUMA_PERIOD_THRESHOLD and local/remote ratio when adjusting
the scan period. The period_slot recalculation can be dropped.

-- 
Mel Gorman
SUSE Labs
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