[Devel] Re: [PATCH v3 04/11] memcg: add lock to synchronize page accounting and migration

[Minchan Kim]

On Tue, Oct 19, 2010 at 9:39 AM, Greg Thelen <gthelen at google.com> wrote:
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
>
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
>
> Introduce a new bit spin lock, PCG_MOVE_LOCK, to synchronize
> the page accounting and migration code.  This reworks the
> locking scheme of _update_stat() and _move_account() by
> adding new lock bit PCG_MOVE_LOCK, which is always taken
> under IRQ disable.
>
> 1. If pages are being migrated from a memcg, then updates to
>   that memcg page statistics are protected by grabbing
>   PCG_MOVE_LOCK using move_lock_page_cgroup().  In an
>   upcoming commit, memcg dirty page accounting will be
>   updating memcg page accounting (specifically: num
>   writeback pages) from IRQ context (softirq).  Avoid a
>   deadlocking nested spin lock attempt by disabling irq on
>   the local processor when grabbing the PCG_MOVE_LOCK.
>
> 2. lock for update_page_stat is used only for avoiding race
>   with move_account().  So, IRQ awareness of
>   lock_page_cgroup() itself is not a problem.  The problem
>   is between mem_cgroup_update_page_stat() and
>   mem_cgroup_move_account_page().
>
> Trade-off:
>  * Changing lock_page_cgroup() to always disable IRQ (or
>    local_bh) has some impacts on performance and I think
>    it's bad to disable IRQ when it's not necessary.
>  * adding a new lock makes move_account() slower.  Score is
>    here.
>
> Performance Impact: moving a 8G anon process.
>
> Before:
>        real    0m0.792s
>        user    0m0.000s
>        sys     0m0.780s
>
> After:
>        real    0m0.854s
>        user    0m0.000s
>        sys     0m0.842s
>
> This score is bad but planned patches for optimization can reduce
> this impact.
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
> Signed-off-by: Greg Thelen <gthelen at google.com>
Reviewed-by: Minchan Kim <minchan.kim at gmail.com>


-- 
Kind regards,
Minchan Kim
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