Re: [PATCH] epoll: avoid calling ep_call_nested() from ep_poll_safewake()

[Davidlohr Bueso]

On Fri, 13 Oct 2017, Jason Baron wrote:

> The ep_poll_safewake() function is used to wakeup potentially nested epoll
> file descriptors. The function uses ep_call_nested() to prevent entering
> the same wake up queue more than once, and to prevent excessively deep
> wakeup paths (deeper than EP_MAX_NESTS). However, this is not necessary
> since we are already preventing these conditions during EPOLL_CTL_ADD. This
> saves extra function calls, and avoids taking a global lock during the
> ep_call_nested() calls.

This makes sense.

> I have, however, left ep_call_nested() for the CONFIG_DEBUG_LOCK_ALLOC
> case, since ep_call_nested() keeps track of the nesting level, and this is
> required by the call to spin_lock_irqsave_nested(). It would be nice to
> remove the ep_call_nested() calls for the CONFIG_DEBUG_LOCK_ALLOC case as
> well, however its not clear how to simply pass the nesting level through
> multiple wake_up() levels without more surgery. In any case, I don't think
> CONFIG_DEBUG_LOCK_ALLOC is generally used for production. This patch, also
> apparently fixes a workload at Google that Salman Qazi reported by
> completely removing the poll_safewake_ncalls->lock from wakeup paths.

I'm a bit curious about the workload (which uses lots of EPOLL_CTL_ADDs) as
I was tackling the nested epoll scaling issue with loop and readywalk lists
in mind. 

> Signed-off-by: Jason Baron <jbaron@xxxxxxxxxx>
> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Salman Qazi <sqazi@xxxxxxxxxx>

Acked-by: Davidlohr Bueso <dbueso@xxxxxxx>

> ---
> fs/eventpoll.c | 47 ++++++++++++++++++-----------------------------
> 1 file changed, 18 insertions(+), 29 deletions(-)

Yay for getting rid of some of the callback hell.

Thanks,
Davidlohr