Re: [PATCH] epoll: remove ep_call_nested() from ep_eventpoll_poll()

[Davidlohr Bueso <dave@stgolabs.net>]

On Tue, 31 Oct 2017, Jason Baron wrote:

>The use of ep_call_nested() in ep_eventpoll_poll(), which is the .poll
>routine for an epoll fd, is used to prevent excessively deep epoll
>nesting, and to prevent circular paths. However, we are already preventing
>these conditions during EPOLL_CTL_ADD. In terms of too deep epoll chains,
>we do in fact allow deep nesting of the epoll fds themselves (deeper
>than EP_MAX_NESTS), however we don't allow more than EP_MAX_NESTS when
>an epoll file descriptor is actually connected to a wakeup source. Thus,
>we do not require the use of ep_call_nested(), since ep_eventpoll_poll(),
>which is called via ep_scan_ready_list() only continues nesting if there
>are events available. Since ep_call_nested() is implemented using a global
>lock, applications that make use of nested epoll can see large performance
>improvements with this change.

Improvements are quite obscene actually, such as for the following epoll_wait()
benchmark with 2 level nesting on a 80 core IvyBridge:

ncpus  vanilla     dirty     delta
1      2447092     3028315   +23.75%
4      231265      2986954   +1191.57%
8      121631      2898796   +2283.27%
16     59749       2902056   +4757.07%
32     26837	   2326314   +8568.30%
64     12926       1341281   +10276.61%

(http://linux-scalability.org/epoll/epoll-test.c)

Thanks,
Davidlohr