linux-kernel - Re: [RFC] eventpoll: Move a kmem_cache_alloc and kmem_cache

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Message-ID: <5240622A.5010305@akamai.com>
Date:	Mon, 23 Sep 2013 11:45:46 -0400
From:	Jason Baron <jbaron@...mai.com>
To:	Eric Wong <normalperson@...t.net>
CC:	Jason Baron <jbaron@...mai.com>, Nathan Zimmer <nzimmer@....com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC] eventpoll: Move a kmem_cache_alloc and kmem_cache_free

On 09/22/2013 04:41 PM, Eric Wong wrote:
> Jason Baron <jbaron@...mai.com> wrote:
>>     epoll: reduce usage of global 'epmutex' lock
>>
>>     Epoll file descriptors that are 1 link from a wakeup source and
>>     are not nested within other epoll descriptors, or pointing to
>>     other epoll descriptors, don't need to check for loop creation or
>>     the creation of wakeup storms. Because of this we can avoid taking
>>     the global 'epmutex' in these cases. This state for the epoll file
>>     descriptor is marked as 'EVENTPOLL_BASIC'. Once the epoll file
>>     descriptor is attached to another epoll file descriptor it is
>>     labeled as 'EVENTPOLL_COMPLEX', and full loop checking and wakeup
>>     storm creation are checked using the the global 'epmutex'. It does
>>     not transition back. Hopefully, this is a common usecase...
> 
> Cool.  I was thinking about doing the same thing down the line (for
> EPOLL_CTL_ADD, too)
> 
>> @@ -166,6 +167,14 @@ struct epitem {
>>  
>>  	/* The structure that describe the interested events and the source fd */
>>  	struct epoll_event event;
>> +
>> +	/* TODO: really necessary? */
>> +	int on_list;
> 
> There's some things we can overload to avoid increasing epitem size
> (.ep, .ffd.fd, ...), so on_list should be unnecessary.

Even with 'on_list' the size of 'epitem' stayed at 128 bytes. Not sure if
there are certain compile options though that can move it over that you
are concerned about...so I think that change is ok.

The biggest hack here was using 'struct rb_node' instead of a proper
'struct rcu_head', so as not to increase the size of epitem. I think this
is safe and I've added build time checks to ensure that 'struct rb_node'
is never smaller than 'struct rcu_head'. But its rather hacky. I will
probably break this change out separately when I re-post so it can be
reviewed independently...

Thanks,

-Jason
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