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SubjectRe: poll()ing a shared pipe triggers unnecessary context switches
On Wed, 15 Dec 2010, Timo Sirainen wrote:

> I have one master process reading status updates from a pipe shared by
> multiple child processes. The child processes also poll() the pipe's
> write fd to see when the master process dies. This works fine, except
> each time a child process writes to the pipe, all the child processes
> wake up in poll()/epoll_wait() and go back to sleep without returning
> anything, triggering unnecessary context switches.
>
> I worked around this by creating yet another pipe just for listening the
> master's death, but this still seems like a Linux bug to me. Solaris and
> BSDs don't behave like this.

Hmm, I *thought* I had already posted this.
The code is in place, in fs/select.c, to filter wakeups based on keys, we
just need to wire up the event-based wakeups to pipes.
See patch below, which should take care of both poll and epoll (and select).




> Test program:
>
> /*
> gcc poller.c -o poller -Wall
>
> Usage: ./poller
> Wait for a while if it prints any "volcs for poll() = .." messages.
> It shouldn't.
> Program stops by hitting enter.
> */
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> #include <unistd.h>
> #include <poll.h>
> #include <sys/time.h>
> #include <sys/resource.h>
>
> void child(int fd)
> {
> struct pollfd pfd;
> struct rusage ru1, ru2;
> int diff;
>
> memset(&pfd, 0, sizeof(pfd));
> pfd.fd = fd;
> pfd.events = 0;
> while (write(fd, "foo\n", 4) == 4) {
> getrusage(RUSAGE_SELF, &ru1);
> if (poll(&pfd, 1, 100) != 0)
> break;
> getrusage(RUSAGE_SELF, &ru2);
>
> diff = ru2.ru_nvcsw - ru1.ru_nvcsw;
> if (diff > 2)
> printf("volcs for poll() = %d\n", diff);
>
> }
> printf("%d exited\n", (int)getpid());
> exit(0);
> }
>
> int main(void)
> {
> struct pollfd pfds[2];
> int i, fd[2];
> char buf[1024];
>
> pipe(fd);
> for (i = 0; i < 10; i++) {
> if (fork() == 0) {
> close(fd[0]);
> child(fd[1]);
> }
> }
>
> memset(pfds, 0, sizeof(pfds));
> pfds[0].fd = 0;
> pfds[0].events = POLLIN;
> pfds[1].fd = fd[0];
> pfds[1].events = POLLIN;
> while (poll(pfds, 2, -1) > 0) {
> if (pfds[0].revents != 0)
> break;
> if (pfds[1].revents != 0)
> read(fd[0], buf, sizeof(buf));
> }
>
> close(fd[0]);
> close(fd[1]);
> printf("done\n");
> return 1;
> }



---
fs/pipe.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)

Index: linux-2.6.mod/fs/pipe.c
===================================================================
--- linux-2.6.mod.orig/fs/pipe.c 2010-12-15 20:49:48.000000000 -0800
+++ linux-2.6.mod/fs/pipe.c 2010-12-15 20:51:42.000000000 -0800
@@ -441,7 +441,7 @@
break;
}
if (do_wakeup) {
- wake_up_interruptible_sync(&pipe->wait);
+ wake_up_interruptible_sync_poll(&pipe->wait, POLLOUT);
kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
}
pipe_wait(pipe);
@@ -450,7 +450,7 @@

/* Signal writers asynchronously that there is more room. */
if (do_wakeup) {
- wake_up_interruptible_sync(&pipe->wait);
+ wake_up_interruptible_sync_poll(&pipe->wait, POLLOUT);
kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
}
if (ret > 0)
@@ -612,7 +612,7 @@
break;
}
if (do_wakeup) {
- wake_up_interruptible_sync(&pipe->wait);
+ wake_up_interruptible_sync_poll(&pipe->wait, POLLIN);
kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
do_wakeup = 0;
}
@@ -623,7 +623,7 @@
out:
mutex_unlock(&inode->i_mutex);
if (do_wakeup) {
- wake_up_interruptible_sync(&pipe->wait);
+ wake_up_interruptible_sync_poll(&pipe->wait, POLLIN);
kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
}
if (ret > 0)
@@ -715,7 +715,7 @@
if (!pipe->readers && !pipe->writers) {
free_pipe_info(inode);
} else {
- wake_up_interruptible_sync(&pipe->wait);
+ wake_up_interruptible_sync_poll(&pipe->wait, POLLIN | POLLOUT);
kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
}


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