linux.git/fs/eventpoll.c, branch v5.11

epoll: add syscall epoll_pwait2

2020-12-19T19:18:38+00:00

Add syscall epoll_pwait2, an epoll_wait variant with nsec resolution that
replaces int timeout with struct timespec.  It is equivalent otherwise.

    int epoll_pwait2(int fd, struct epoll_event *events,
                     int maxevents,
                     const struct timespec *timeout,
                     const sigset_t *sigset);

The underlying hrtimer is already programmed with nsec resolution.
pselect and ppoll also set nsec resolution timeout with timespec.

The sigset_t in epoll_pwait has a compat variant. epoll_pwait2 needs
the same.

For timespec, only support this new interface on 2038 aware platforms
that define __kernel_timespec_t. So no CONFIG_COMPAT_32BIT_TIME.

Link: https://lkml.kernel.org/r/20201121144401.3727659-3-willemdebruijn.kernel@gmail.com
Signed-off-by: Willem de Bruijn 
Cc: Al Viro 
Cc: Arnd Bergmann 
Cc: Matthew Wilcox (Oracle) 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

epoll: convert internal api to timespec64

2020-12-19T19:18:38+00:00

Patch series "add epoll_pwait2 syscall", v4.

Enable nanosecond timeouts for epoll.

Analogous to pselect and ppoll, introduce an epoll_wait syscall
variant that takes a struct timespec instead of int timeout.

This patch (of 4):

Make epoll more consistent with select/poll: pass along the timeout as
timespec64 pointer.

In anticipation of additional changes affecting all three polling
mechanisms:

- add epoll_pwait2 syscall with timespec semantics,
  and share poll_select_set_timeout implementation.
- compute slack before conversion to absolute time,
  to save one ktime_get_ts64 call.

Link: https://lkml.kernel.org/r/20201121144401.3727659-1-willemdebruijn.kernel@gmail.com
Link: https://lkml.kernel.org/r/20201121144401.3727659-2-willemdebruijn.kernel@gmail.com
Signed-off-by: Willem de Bruijn 
Cc: Al Viro 
Cc: Matthew Wilcox (Oracle) 
Cc: Arnd Bergmann 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

epoll: eliminate unnecessary lock for zero timeout

2020-12-19T19:18:38+00:00

We call ep_events_available() under lock when timeout is 0, and then call
it without locks in the loop for the other cases.

Instead, call ep_events_available() without lock for all cases.  For
non-zero timeouts, we will recheck after adding the thread to the wait
queue.  For zero timeout cases, by definition, user is opportunistically
polling and will have to call epoll_wait again in the future.

Note that this lock was kept in c5a282e9635e9 because the whole loop was
historically under lock.

This patch results in a 1% CPU/RPC reduction in RPC benchmarks.

Link: https://lkml.kernel.org/r/20201106231635.3528496-9-soheil.kdev@gmail.com
Signed-off-by: Soheil Hassas Yeganeh 
Suggested-by: Eric Dumazet 
Reviewed-by: Eric Dumazet 
Reviewed-by: Willem de Bruijn 
Reviewed-by: Khazhismel Kumykov 
Cc: Guantao Liu 
Cc: Linus Torvalds 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

epoll: replace gotos with a proper loop

2020-12-19T19:18:38+00:00

The existing loop is pointless, and the labels make it really hard to
follow the structure.

Replace that control structure with a simple loop that returns when there
are new events, there is a signal, or the thread has timed out.

Link: https://lkml.kernel.org/r/20201106231635.3528496-8-soheil.kdev@gmail.com
Signed-off-by: Soheil Hassas Yeganeh 
Suggested-by: Linus Torvalds 
Reviewed-by: Eric Dumazet 
Reviewed-by: Willem de Bruijn 
Reviewed-by: Khazhismel Kumykov 
Cc: Guantao Liu 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

epoll: pull all code between fetch_events and send_event into the loop

2020-12-19T19:18:38+00:00

This is a no-op change which simplifies the follow up patches.

Link: https://lkml.kernel.org/r/20201106231635.3528496-7-soheil.kdev@gmail.com
Signed-off-by: Soheil Hassas Yeganeh 
Suggested-by: Linus Torvalds 
Reviewed-by: Eric Dumazet 
Reviewed-by: Willem de Bruijn 
Reviewed-by: Khazhismel Kumykov 
Cc: Guantao Liu 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

epoll: simplify and optimize busy loop logic

2020-12-19T19:18:37+00:00

ep_events_available() is called multiple times around the busy loop logic,
even though the logic is generally not used.  ep_reset_busy_poll_napi_id()
is similarly always called, even when busy loop is not used.

Eliminate ep_reset_busy_poll_napi_id() and inline it inside
ep_busy_loop().  Make ep_busy_loop() return whether there are any events
available after the busy loop.  This will eliminate unnecessary loads and
branches, and simplifies the loop.

Link: https://lkml.kernel.org/r/20201106231635.3528496-6-soheil.kdev@gmail.com
Signed-off-by: Soheil Hassas Yeganeh 
Reviewed-by: Eric Dumazet 
Reviewed-by: Willem de Bruijn 
Reviewed-by: Khazhismel Kumykov 
Cc: Guantao Liu 
Cc: Linus Torvalds 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

epoll: move eavail next to the list_empty_careful check

2020-12-19T19:18:37+00:00

This is a no-op change and simply to make the code more coherent.

Link: https://lkml.kernel.org/r/20201106231635.3528496-5-soheil.kdev@gmail.com
Signed-off-by: Soheil Hassas Yeganeh 
Suggested-by: Linus Torvalds 
Reviewed-by: Eric Dumazet 
Reviewed-by: Willem de Bruijn 
Reviewed-by: Khazhismel Kumykov 
Cc: Guantao Liu 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

epoll: pull fatal signal checks into ep_send_events()

2020-12-19T19:18:37+00:00

To simplify the code, pull in checking the fatal signals into
ep_send_events().  ep_send_events() is called only from ep_poll().

Note that, previously, we were always checking fatal events, but it is
checked only if eavail is true.  This should be fine because the goal of
that check is to quickly return from epoll_wait() when there is a pending
fatal signal.

Link: https://lkml.kernel.org/r/20201106231635.3528496-4-soheil.kdev@gmail.com
Signed-off-by: Soheil Hassas Yeganeh 
Suggested-by: Willem de Bruijn 
Reviewed-by: Eric Dumazet 
Reviewed-by: Willem de Bruijn 
Reviewed-by: Khazhismel Kumykov 
Cc: Guantao Liu 
Cc: Linus Torvalds 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

epoll: simplify signal handling

2020-12-19T19:18:37+00:00

Check signals before locking ep->lock, and immediately return -EINTR if
there is any signal pending.

This saves a few loads, stores, and branches from the hot path and
simplifies the loop structure for follow up patches.

Link: https://lkml.kernel.org/r/20201106231635.3528496-3-soheil.kdev@gmail.com
Signed-off-by: Soheil Hassas Yeganeh 
Suggested-by: Linus Torvalds 
Reviewed-by: Eric Dumazet 
Reviewed-by: Willem de Bruijn 
Reviewed-by: Khazhismel Kumykov 
Cc: Guantao Liu 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

epoll: check for events when removing a timed out thread from the wait queue

2020-12-19T19:18:37+00:00

Patch series "simplify ep_poll".

This patch series is a followup based on the suggestions and feedback by
Linus:
https://lkml.kernel.org/r/CAHk-=wizk=OxUyQPbO8MS41w2Pag1kniUV5WdD5qWL-gq1kjDA@mail.gmail.com

The first patch in the series is a fix for the epoll race in presence of
timeouts, so that it can be cleanly backported to all affected stable
kernels.

The rest of the patch series simplify the ep_poll() implementation. Some
of these simplifications result in minor performance enhancements as well.
We have kept these changes under self tests and internal benchmarks for a
few days, and there are minor (1-2%) performance enhancements as a result.

This patch (of 8):

After abc610e01c66 ("fs/epoll: avoid barrier after an epoll_wait(2)
timeout"), we break out of the ep_poll loop upon timeout, without checking
whether there is any new events available. Prior to that patch-series we
always called ep_events_available() after exiting the loop.

This can cause races and missed wakeups. For example, consider the
following scenario reported by Guantao Liu:

Suppose we have an eventfd added using EPOLLET to an epollfd.

Thread 1: Sleeps for just below 5ms and then writes to an eventfd.
Thread 2: Calls epoll_wait with a timeout of 5 ms. If it sees an
event of the eventfd, it will write back on that fd.
Thread 3: Calls epoll_wait with a negative timeout.

Prior to abc610e01c66, it is guaranteed that Thread 3 will wake up either
by Thread 1 or Thread 2. After abc610e01c66, Thread 3 can be blocked
indefinitely if Thread 2 sees a timeout right before the write to the
eventfd by Thread 1. Thread 2 will be woken up from
schedule_hrtimeout_range and, with evail 0, it will not call
ep_send_events().

To fix this issue:
1) Simplify the timed_out case as suggested by Linus.
2) while holding the lock, recheck whether the thread was woken up
after its time out has reached.

Note that (2) is different from Linus' original suggestion: It do not set
"eavail = ep_events_available(ep)" to avoid unnecessary contention (when
there are too many timed-out threads and a small number of events), as
well as races mentioned in the discussion thread.

This is the first patch in the series so that the backport to stable
releases is straightforward.

Link: https://lkml.kernel.org/r/20201106231635.3528496-1-soheil.kdev@gmail.com
Link: https://lkml.kernel.org/r/CAHk-=wizk=OxUyQPbO8MS41w2Pag1kniUV5WdD5qWL-gq1kjDA@mail.gmail.com
Link: https://lkml.kernel.org/r/20201106231635.3528496-2-soheil.kdev@gmail.com
Fixes: abc610e01c66 ("fs/epoll: avoid barrier after an epoll_wait(2) timeout")
Signed-off-by: Soheil Hassas Yeganeh
Tested-by: Guantao Liu
Suggested-by: Linus Torvalds
Reported-by: Guantao Liu
Reviewed-by: Eric Dumazet
Reviewed-by: Willem de Bruijn
Reviewed-by: Khazhismel Kumykov
Reviewed-by: Davidlohr Bueso
Signed-off-by: Andrew Morton
Signed-off-by: Linus Torvalds