[patch 47/86] fs/epoll: remove unnecessary wakeups of nested epoll

Andrew Morton
2019-12-05T00:52:18Z

From: Heiher <r@hev.cc>
Subject: fs/epoll: remove unnecessary wakeups of nested epoll

Take the case where we have:

t0
| (ew)
e0
| (et)
e1
| (lt)
s0

t0: thread 0
e0: epoll fd 0
e1: epoll fd 1
s0: socket fd 0
ew: epoll_wait
et: edge-trigger
lt: level-trigger

We remove unnecessary wakeups to prevent the nested epoll that working in edge-
triggered mode to waking up continuously.

Test code:
#include <unistd.h>
#include <sys/epoll.h>
#include <sys/socket.h>

int main(int argc, char *argv[])
{
int sfd[2];
int efd[2];
struct epoll_event e;

if (socketpair(AF_UNIX, SOCK_STREAM, 0, sfd) < 0)
goto out;

efd[0] = epoll_create(1);
if (efd[0] < 0)
goto out;

efd[1] = epoll_create(1);
if (efd[1] < 0)
goto out;

e.events = EPOLLIN;
if (epoll_ctl(efd[1], EPOLL_CTL_ADD, sfd[0], &e) < 0)
goto out;

e.events = EPOLLIN | EPOLLET;
if (epoll_ctl(efd[0], EPOLL_CTL_ADD, efd[1], &e) < 0)
goto out;

if (write(sfd[1], "w", 1) != 1)
goto out;

if (epoll_wait(efd[0], &e, 1, 0) != 1)
goto out;

if (epoll_wait(efd[0], &e, 1, 0) != 0)
goto out;

close(efd[0]);
close(efd[1]);
close(sfd[0]);
close(sfd[1]);

return 0;

out:
return -1;
}

More tests:
https://github.com/heiher/epoll-wakeup

Link: http://lkml.kernel.org/r/20191009060516.3577-1-r@hev.cc
Signed-off-by: hev <r@hev.cc>
Reviewed-by: Roman Penyaev <rpenyaev@suse.de>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Eric Wong <e@80x24.org>
Cc: Jason Baron <jbaron@akamai.com>
Cc: Sridhar Samudrala <sridhar.samudrala@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

fs/eventpoll.c | 16 ----------------
1 file changed, 16 deletions(-)

--- a/fs/eventpoll.c~fs-epoll-remove-unnecessary-wakeups-of-nested-epoll
+++ a/fs/eventpoll.c
@@ -666,7 +666,6 @@ static __poll_t ep_scan_ready_list(struc
void *priv, int depth, bool ep_locked)
{
__poll_t res;
- int pwake = 0;
struct epitem *epi, *nepi;
LIST_HEAD(txlist);

@@ -733,26 +732,11 @@ static __poll_t ep_scan_ready_list(struc
*/
list_splice(&txlist, &ep->rdllist);
__pm_relax(ep->ws);
-
- if (!list_empty(&ep->rdllist)) {
- /*
- * Wake up (if active) both the eventpoll wait list and
- * the ->poll() wait list (delayed after we release the lock).
- */
- if (waitqueue_active(&ep->wq))
- wake_up(&ep->wq);
- if (waitqueue_active(&ep->poll_wait))
- pwake++;
- }
write_unlock_irq(&ep->lock);

if (!ep_locked)
mutex_unlock(&ep->mtx);

- /* We have to call this outside the lock */
- if (pwake)
- ep_poll_safewake(&ep->poll_wait);
-
return res;
}

_