From: Davidlohr Bueso <dave@xxxxxxxxxxxx> Subject: fs/epoll: deal with wait_queue only once There is no reason why we rearm the waitiqueue upon every fetch_events retry (for when events are found yet send_events() fails). If nothing else, this saves four lock operations per retry, and furthermore reduces the scope of the lock even further. [akpm@xxxxxxxxxxxxxxxxxxxx: restore code to original position, fix and reflow comment] Link: http://lkml.kernel.org/r/20181114182532.27981-2-dave@xxxxxxxxxxxx Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx> Cc: Jason Baron <jbaron@xxxxxxxxxx> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- fs/eventpoll.c | 29 ++++++++++++++++++----------- 1 file changed, 18 insertions(+), 11 deletions(-) --- a/fs/eventpoll.c~fs-epoll-deal-with-wait_queue-only-once +++ a/fs/eventpoll.c @@ -1749,6 +1749,7 @@ static int ep_poll(struct eventpoll *ep, { int res = 0, eavail, timed_out = 0; u64 slack = 0; + bool waiter = false; wait_queue_entry_t wait; ktime_t expires, *to = NULL; @@ -1794,14 +1795,18 @@ fetch_events: ep_reset_busy_poll_napi_id(ep); /* - * We don't have any available event to return to the caller. - * We need to sleep here, and we will be wake up by - * ep_poll_callback() when events will become available. + * We don't have any available event to return to the caller. We need + * to sleep here, and we will be woken by ep_poll_callback() when events + * become available. */ - init_waitqueue_entry(&wait, current); - spin_lock_irq(&ep->wq.lock); - __add_wait_queue_exclusive(&ep->wq, &wait); - spin_unlock_irq(&ep->wq.lock); + if (!waiter) { + waiter = true; + init_waitqueue_entry(&wait, current); + + spin_lock_irq(&ep->wq.lock); + __add_wait_queue_exclusive(&ep->wq, &wait); + spin_unlock_irq(&ep->wq.lock); + } for (;;) { /* @@ -1837,10 +1842,6 @@ fetch_events: __set_current_state(TASK_RUNNING); - spin_lock_irq(&ep->wq.lock); - __remove_wait_queue(&ep->wq, &wait); - spin_unlock_irq(&ep->wq.lock); - send_events: /* * Try to transfer events to user space. In case we get 0 events and @@ -1851,6 +1852,12 @@ send_events: !(res = ep_send_events(ep, events, maxevents)) && !timed_out) goto fetch_events; + if (waiter) { + spin_lock_irq(&ep->wq.lock); + __remove_wait_queue(&ep->wq, &wait); + spin_unlock_irq(&ep->wq.lock); + } + return res; } _
