linux-kernel - Re: [PATCH 04/10] memcg: disable local interrupts in lock_page

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Message-ID: <20101012053935.GD25875@balbir.in.ibm.com>
Date:	Tue, 12 Oct 2010 11:09:35 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Greg Thelen <gthelen@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	containers@...ts.osdl.org, Andrea Righi <arighi@...eler.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH 04/10] memcg: disable local interrupts in
 lock_page_cgroup()

* Greg Thelen <gthelen@...gle.com> [2010-10-03 23:57:59]:

> If pages are being migrated from a memcg, then updates to that
> memcg's page statistics are protected by grabbing a bit spin lock
> using lock_page_cgroup().  In an upcoming commit memcg dirty page
> accounting will be updating memcg page accounting (specifically:
> num writeback pages) from softirq.  Avoid a deadlocking nested
> spin lock attempt by disabling interrupts on the local processor
> when grabbing the page_cgroup bit_spin_lock in lock_page_cgroup().
> This avoids the following deadlock:
> statistic
>       CPU 0             CPU 1
>                     inc_file_mapped
>                     rcu_read_lock
>   start move
>   synchronize_rcu
>                     lock_page_cgroup
>                       softirq
>                       test_clear_page_writeback
>                       mem_cgroup_dec_page_stat(NR_WRITEBACK)
>                       rcu_read_lock
>                       lock_page_cgroup   /* deadlock */
>                       unlock_page_cgroup
>                       rcu_read_unlock
>                     unlock_page_cgroup
>                     rcu_read_unlock
> 
> By disabling interrupts in lock_page_cgroup, nested calls
> are avoided.  The softirq would be delayed until after inc_file_mapped
> enables interrupts when calling unlock_page_cgroup().
> 
> The normal, fast path, of memcg page stat updates typically
> does not need to call lock_page_cgroup(), so this change does
> not affect the performance of the common case page accounting.
> 
> Signed-off-by: Andrea Righi <arighi@...eler.com>
> Signed-off-by: Greg Thelen <gthelen@...gle.com>
-- 


It will take more convincing, all important functions (charge/uncharge
use lock_page_cgroup()). I'd like to see the page fault scalability
test results. I am not against this patch, just want to see the
scalability numbers.

	Three Cheers,
	Balbir
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