linux-kernel - Re: [PATCH 2/2 v2] epoll: Do not take global 'epmutex' for simple topologies

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Date:	Fri, 04 Oct 2013 11:16:12 -0400
From:	Jason Baron <jbaron@...mai.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	"normalperson@...t.net" <normalperson@...t.net>,
	"nzimmer@....com" <nzimmer@....com>,
	"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
	"nelhage@...hage.com" <nelhage@...hage.com>,
	"davidel@...ilserver.org" <davidel@...ilserver.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 2/2 v2] epoll: Do not take global 'epmutex' for simple
 topologies

On 10/03/2013 05:50 PM, Andrew Morton wrote:
> On Tue,  1 Oct 2013 17:08:14 +0000 (GMT) Jason Baron <jbaron@...mai.com> wrote:
>
>> When calling EPOLL_CTL_ADD for an epoll file descriptor that is attached
>> directly to a wakeup source, we do not need to take the global 'epmutex',
>> unless the epoll file descriptor is nested. The purpose of taking
>> the 'epmutex' on add is to prevent complex topologies such as loops and
>> deep wakeup paths from forming in parallel through multiple EPOLL_CTL_ADD
>> operations. However, for the simple case of an epoll file descriptor
>> attached directly to a wakeup source (with no nesting), we do not need
>> to hold the 'epmutex'.
>>
>> This patch along with 'epoll: optimize EPOLL_CTL_DEL using rcu' improves
>> scalability on larger systems. Quoting Nathan Zimmer's mail on SPECjbb
>> performance:
>>
>> "
>> On the 16 socket run the performance went from 35k jOPS to 125k jOPS.
>> In addition the benchmark when from scaling well on 10 sockets to scaling well
>> on just over 40 sockets.
>>
>> ...
>>
>> Currently the benchmark stops scaling at around 40-44 sockets but it seems like
>> I found a second unrelated bottleneck.
>> "
> I couldn't resist fiddling.  Please review
>
> From: Andrew Morton <akpm@...ux-foundation.org>
> Subject: epoll-do-not-take-global-epmutex-for-simple-topologies-fix
>
> - use `bool' for boolean variables
> - remove unneeded/undesirable cast of void*
> - add missed ep_scan_ready_list() kerneldoc 

Hi Andrew,

Looks good to me. Feel free to add:

Reviewed-by: Jason Baron <jbaron@...mai.com>

Thanks,

-Jason
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