linux-kernel - Re: [PATCH] fix mem_cgroup_split_huge

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Date:	Mon, 21 Nov 2011 16:30:11 +0530
From:	Balbir Singh <bsingharora@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>, cgroups@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"hannes@...xchg.org" <hannes@...xchg.org>, mhocko@...e.cz,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH] fix mem_cgroup_split_huge_fixup to work efficiently.

On Thu, Nov 17, 2011 at 7:03 AM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@...fujitsu.com> wrote:
>
> I'll send this again when mm is shipped.
> I sometimes see mem_cgroup_split_huge_fixup() in perf report and noticed
> it's very slow. This fixes it. Any comments are welcome.
>

How do you see this - what tests?

> ==
> Subject: [PATCH] fix mem_cgroup_split_huge_fixup to work efficiently.
>
> at split_huge_page(), mem_cgroup_split_huge_fixup() is called to
> handle page_cgroup modifcations. It takes move_lock_page_cgroup()
> and modify page_cgroup and LRU accounting jobs and called
> HPAGE_PMD_SIZE - 1 times.
>
> But thinking again,
>  - compound_lock() is held at move_accout...then, it's not necessary
>    to take move_lock_page_cgroup().
>  - LRU is locked and all tail pages will go into the same LRU as
>    head is now on.
>  - page_cgroup is contiguous in huge page range.
>
> This patch fixes mem_cgroup_split_huge_fixup() as to be called once per
> hugepage and reduce costs for spliting.

The change seems reasonable, I am working on a test setup and hope to
test it soon

Balbir
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