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

[Daisuke Nishimura]

On Mon, 18 Oct 2010 17:39:37 -0700
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>

This approach is more straightforward and easy to understand.

Acked-by: Daisuke Nishimura <nishimura at mxp.nes.nec.co.jp>
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