[Devel] Re: [PATCH 04/10] memcg: disable local interrupts in lock_page_cgroup()

[Balbir Singh]

* Greg Thelen <gthelen at google.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 at develer.com>
> Signed-off-by: Greg Thelen <gthelen at google.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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