On Thu, 19 Jan 2012 15:47:12 +0100
Michal Hocko <mhocko@xxxxxxx> wrote:
> On Fri 13-01-12 17:45:10, KAMEZAWA Hiroyuki wrote:
> > From 3df71cef5757ee6547916c4952f04a263c1b8ddb Mon Sep 17 00:00:00 2001
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> > Date: Fri, 13 Jan 2012 17:07:35 +0900
> > Subject: [PATCH 7/7] memcg: make mem_cgroup_begin_update_stat to use global pcpu.
> >
> > Now, a per-cpu flag to show the memcg is under account moving is
> > now implemented as per-memcg-per-cpu.
> >
> > So, when accessing this, we need to access memcg 1st. But this
> > function is called even when status update doesn't occur. Then,
> > accessing struct memcg is an overhead in such case.
> >
> > This patch removes per-cpu-per-memcg MEM_CGROUP_ON_MOVE and add
> > per-cpu vairable to do the same work. For per-memcg, atomic
> > counter is added. By this, mem_cgroup_begin_update_stat() will
> > just access percpu variable in usual case and don't need to find & access
> > memcg. This reduces overhead.
>
> I agree that move_account is not a hotpath and that we don't have
> to optimize for it but I guess we can do better. If we use a cookie
> parameter for
> mem_cgroup_{begin,end}_update_stat(struct page *page, unsigned long *cookie)
> then we can stab page_cgroup inside and use the last bit for
> locked. Then we do not have to call lookup_page_cgroup again in
> mem_cgroup_update_page_stat and just replace page by the cookie.
> What do you think?
>
Because these routine is called as
mem_cgroup_begin_update_stat()
if (condition)
set_page_flag
mem_cgroup_update_stat()
mem_cgroup_end_update_stat()
In earlier version(not posted), I did so. Now, I don't because of 2 reasons.
1. I wonder it's better not to have extra arguments in begin_xxx it
will be overhead itself.
2. my work's final purpose is integrate page_cgroup to struct page.
If I can do, lookup_page_cgroup() cost will be almost 0 and we'll revert
the cookie, finally.
So, can't we keep this update routine simple for a while ?
If we saw it's finally impossible to integrate page_cgroup to page,
I'd like to consider 'cookie' again.
BTW, If we use spinlock and need to do irq_disable() in begin_update_stat()
we'll need to pass *flags...
Thanks,
-Kame
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