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

Daisuke Nishimura
2010-10-19T05:08:45Z

On Mon, 18 Oct 2010 17:39:37 -0700
Greg Thelen <gthelen@google.com> wrote:

> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@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@jp.fujitsu.com>
> Signed-off-by: Greg Thelen <gthelen@google.com>

This approach is more straightforward and easy to understand.

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