linux-kernel - Re: [PATCH 3/3] mm: when handling percpu_pagelist_fraction, use on_each

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Message-ID: <CAOtvUMdT0-oQMTsHAjFqL6K8vrLeCcXG2hX-sShxu6GGRBPxJw@mail.gmail.com>
Date:	Mon, 8 Apr 2013 15:20:44 +0300
From:	Gilad Ben-Yossef <gilad@...yossef.com>
To:	Cody P Schafer <cody@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Linux MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] mm: when handling percpu_pagelist_fraction, use
 on_each_cpu() to set percpu pageset fields.

On Fri, Apr 5, 2013 at 11:33 PM, Cody P Schafer <cody@...ux.vnet.ibm.com> wrote:
> In free_hot_cold_page(), we rely on pcp->batch remaining stable.
> Updating it without being on the cpu owning the percpu pageset
> potentially destroys this stability.
>
> Change for_each_cpu() to on_each_cpu() to fix.

Are you referring to this? -

1329         if (pcp->count >= pcp->high) {
1330                 free_pcppages_bulk(zone, pcp->batch, pcp);
1331                 pcp->count -= pcp->batch;
1332         }

I'm probably missing the obvious but won't it be simpler to do this in
 free_hot_cold_page() -

1329         if (pcp->count >= pcp->high) {
1330                  unsigned int batch = ACCESS_ONCE(pcp->batch);
1331                 free_pcppages_bulk(zone, batch, pcp);
1332                 pcp->count -= batch;
1333         }

Now the batch value used is stable and you don't have to IPI every CPU
in the system just to change a config knob...

Thanks,
Gilad



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