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SubjectRe: ppoll() stuck on POLLIN while TCP peer is sending
(changing Cc:)

Eric Wong <normalperson@yhbt.net> wrote:
> I'm finding ppoll() unexpectedly stuck when waiting for POLLIN on a
> local TCP socket. The isolated code below can reproduces the issue
> after many minutes (<1 hour). It might be easier to reproduce on
> a busy system while disk I/O is happening.

s/might be/is/

Strangely, I've bisected this seemingly networking-related issue down to
the following commit:

commit 1fb3f8ca0e9222535a39b884cb67a34628411b9f
Author: Mel Gorman <mgorman@suse.de>
Date: Mon Oct 8 16:29:12 2012 -0700

mm: compaction: capture a suitable high-order page immediately when it is made available

That commit doesn't revert cleanly on v3.7.1, and I don't feel
comfortable touching that code myself.

Instead, I disabled THP+compaction under v3.7.1 and I've been unable to
reproduce the issue without THP+compaction.

As I mention in http://mid.gmane.org/20121229113434.GA13336@dcvr.yhbt.net
I run my below test (`toosleepy') with heavy network and disk activity
for a long time before hitting this.

My disk activity involves copying large files around to different local
drives over loopback[1], so perhaps the duplicate pages get compacted
away? toosleepy also reuses the same 16K junk data all around.


[1] my full setup is very strange.

Other than the FUSE component I forgot to mention, little depends on
the kernel. With all this, the standalone toosleepy can get stuck.
I'll try to reproduce it with less...

(possibly relevant info, I don't expect you to duplicate my setup
as it requires many, many patched userspace components :x):

fusedav (with many bugfixes[2]) -> (FUSE device)
zbatery (Ruby 1.9.3-p362) -> omgdav (in zbatery process) -> (TCP)
MogileFS (patched[3]) -> (TCP)
cmogstored

The (zbatery -> omgdav -> MogileFS -> cmogstored) path is all userspace.
cmogstored uses sendfile and may talk to itself via MogileFS replication:
MogileFS(replicate) -> HTTP GET from cmogstored -> HTTP PUT to cmogstored

(MFS was designed for clusters, but I only have one machine right
now) MogileFS replicate does not use splice between sockets, just
read/write, cmogstored does not use splice (yet) either.

The stuck ppoll() I noticed is from Ruby (zbatery/omgdav) while the
send() was from fusedav (using neon).

[2] my patches on http://bugs.debian.org/fusedav and
git clone git://bogomips.org/fusedav.git home
[3] git clone git://bogomips.org/MogileFS-Server.git testing

> This may also be related to an epoll-related issue reported
> by Andreas Voellmy:
> http://thread.gmane.org/gmane.linux.kernel/1408782/

(That epoll issue was unrelated and fixed while I was hunting this bug)

> My example involves a 3 thread data flow between two pairs
> of (4) sockets:
>
> send_loop -> recv_loop(recv_send) -> recv_loop(recv_only)
> pair_a[1] -> (pair_a[0] -> pair_b[1]) -> pair_b[0]
>
> At least 3.7 and 3.7.1 are affected.
>
> I have tcp_low_latency=1 set, I will try 0 later
>
> The last progress message I got was after receiving 2942052597760
> bytes on fd=7 (out of 64-bit ULONG_MAX / 2)
>
> strace:
>
> 3644 sendto(4, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 16384, 0, NULL, 0 <unfinished ...>
> 3643 sendto(6, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 16384, 0, NULL, 0 <unfinished ...>
> 3642 ppoll([{fd=7, events=POLLIN}], 1, NULL, NULL, 8 <unfinished ...>
> 3641 futex(0x7f23ed8129d0, FUTEX_WAIT, 3644, NULL <unfinished ...>
>
> The first and last lines of the strace are expected:
>
> + 3644 sendto(4) is blocked because 3643 is blocked on sendto(fd=6)
> and not able to call recv().
> + 3641 is the main thread calling pthread_join
>
> What is unexpected is the tid=3643 and tid=3642 interaction. As confirmed
> by lsof below, fd=6 is sending to wake up fd=7, but ppoll(fd=7) seems
> to not be waking up.
>
> lsof:
> toosleepy 3641 ew 4u IPv4 12405 0t0 TCP localhost:55904->localhost:33249 (ESTABLISHED)
> toosleepy 3641 ew 5u IPv4 12406 0t0 TCP localhost:33249->localhost:55904 (ESTABLISHED)
> toosleepy 3641 ew 6u IPv4 12408 0t0 TCP localhost:48777->localhost:33348 (ESTABLISHED)
> toosleepy 3641 ew 7u IPv4 12409 0t0 TCP localhost:33348->localhost:48777 (ESTABLISHED)
>
> System info: Linux 3.7.1 x86_64 SMP PREEMPT
> AMD Phenom(tm) II X4 945 Processor (4 cores)
> Nothing interesting in dmesg, iptables rules are empty.
>
> I have not yet been able to reproduce the issue using UNIX sockets,
> only TCP, but you can run:
>
> ./toosleepy unix
>
> ...to test with UNIX sockets intead of TCP.
>
> The following code is also available via git://bogomips.org/toosleepy
> gcc -o toosleepy -O2 -Wall -lpthread toosleepy.c
> -------------------------------- 8< ------------------------------------
> #define _GNU_SOURCE
> #include <poll.h>
> #include <sys/ioctl.h>
> #include <pthread.h>
> #include <sys/types.h>
> #include <sys/socket.h>
> #include <arpa/inet.h>
> #include <netinet/tcp.h>
> #include <stdint.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <errno.h>
> #include <string.h>
> #include <unistd.h>
> #include <fcntl.h>
> #include <assert.h>
> #include <limits.h>
>
> struct receiver {
> int rfd;
> int sfd;
> };
>
> /* blocking sender */
> static void * send_loop(void *fdp)
> {
> int fd = *(int *)fdp;
> char buf[16384];
> ssize_t s;
> size_t sent = 0;
> size_t max = (size_t)ULONG_MAX / 2;
>
> while (sent < max) {
> s = send(fd, buf, sizeof(buf), 0);
> if (s > 0)
> sent += s;
> if (s == -1)
> assert(errno == EINTR);
> }
> dprintf(2, "%d done sending: %zu\n", fd, sent);
> close(fd);
> return NULL;
> }
>
> /* non-blocking receiver, using ppoll */
> static void * recv_loop(void *p)
> {
> const struct receiver *rcvr = p;
> char buf[16384];
> nfds_t nfds = 1;
> struct pollfd fds;
> int rc;
> ssize_t r, s;
> size_t received = 0;
> size_t sent = 0;
>
> for (;;) {
> r = recv(rcvr->rfd, buf, sizeof(buf), 0);
> if (r == 0) {
> break;
> } else if (r == -1) {
> assert(errno == EAGAIN);
>
> fds.fd = rcvr->rfd;
> fds.events = POLLIN;
> errno = 0;
> rc = ppoll(&fds, nfds, NULL, NULL);
> assert(rc == 1);
> } else {
> assert(r > 0);
> received += r;
> if (rcvr->sfd >= 0) {
> s = send(rcvr->sfd, buf, sizeof(buf), 0);
> if (s > 0)
> sent += s;
> if (s == -1)
> assert(errno == EINTR);
> } else {
> /* just burn some cycles */
> write(-1, buf, sizeof(buf));
> }
> }
> if ((received % (sizeof(buf) * sizeof(buf) * 16) == 0))
> dprintf(2, " %d progress: %zu\n",
> rcvr->rfd, received);
> }
> dprintf(2, "%d got: %zu\n", rcvr->rfd, received);
> if (rcvr->sfd >= 0) {
> dprintf(2, "%d sent: %zu\n", rcvr->sfd, sent);
> close(rcvr->sfd);
> }
>
> return NULL;
> }
>
> static void tcp_socketpair(int sv[2], int accept_flags)
> {
> struct sockaddr_in addr;
> socklen_t addrlen = sizeof(addr);
> int l = socket(PF_INET, SOCK_STREAM, 0);
> int c = socket(PF_INET, SOCK_STREAM, 0);
> int a;
>
> addr.sin_family = AF_INET;
> addr.sin_addr.s_addr = INADDR_ANY;
> addr.sin_port = 0;
> assert(0 == bind(l, (struct sockaddr*)&addr, addrlen));
> assert(0 == listen(l, 1024));
> assert(0 == getsockname(l, (struct sockaddr *)&addr, &addrlen));
> assert(0 == connect(c, (struct sockaddr *)&addr, addrlen));
> a = accept4(l, NULL, NULL, accept_flags);
> assert(a >= 0);
> close(l);
> sv[0] = a;
> sv[1] = c;
> }
>
> int main(int argc, char *argv[])
> {
> int pair_a[2];
> int pair_b[2];
> pthread_t s, rs, r;
> struct receiver recv_only;
> struct receiver recv_send;
>
> if (argc == 2 && strcmp(argv[1], "unix") == 0) {
> int val;
> assert(0 == socketpair(AF_UNIX, SOCK_STREAM, 0, pair_a));
> assert(0 == socketpair(AF_UNIX, SOCK_STREAM, 0, pair_b));
> /* only make the receiver non-blocking */
> val = 1;
> assert(0 == ioctl(pair_a[0], FIONBIO, &val));
> val = 1;
> assert(0 == ioctl(pair_b[0], FIONBIO, &val));
> } else {
> tcp_socketpair(pair_a, SOCK_NONBLOCK);
> tcp_socketpair(pair_b, SOCK_NONBLOCK);
> }
>
> recv_send.rfd = pair_a[0];
> recv_send.sfd = pair_b[1];
> recv_only.rfd = pair_b[0];
> recv_only.sfd = -1;
>
> /*
> * data flow:
> * send_loop -> recv_loop(recv_send) -> recv_loop(recv_only)
> * pair_a[1] -> (pair_a[0] -> pair_b[1]) -> pair_b[0]
> */
> assert(0 == pthread_create(&r, NULL, recv_loop, &recv_only));
> assert(0 == pthread_create(&rs, NULL, recv_loop, &recv_send));
> assert(0 == pthread_create(&s, NULL, send_loop, &pair_a[1]));
> assert(0 == pthread_join(s, NULL));
> assert(0 == pthread_join(rs, NULL));
> assert(0 == pthread_join(r, NULL));
>
> return 0;
> }
> -------------------------------- 8< ------------------------------------
> Any help/suggestions/test patches would be greatly appreciated.
> Thanks for reading!
>
> --
> Eric Wong


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