Re: [RFC] eventpoll: Move a kmem_cache_alloc and kmem_cache_free

[Eric Wong <normalperson@yhbt.net>]

Jason Baron <jbaron@akamai.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.