Hi,
I've run into a situation that seems like a bug. I'm using qemu 1.4.2
(with additional patches) from within openstack.
I'm using virtio-serial-pci to provide a channel between the guest and host.
On occasion when doing suspend/resume I run into a case where the main
qemu thread ends up chewing 100% of a cpu.
I attached strace to the thread and it showed qemu just spitting messages:
write(35, "HRBT\0\1\0\3d<\230k\0\0\0\0\0\0\1\330\0\0\0\0enqueue\0"...,
472) = -1 EAGAIN (Resource temporarily unavailable)
write(35, "HRBT\0\1\0\3d<\230k\0\0\0\0\0\0\1\330\0\0\0\0enqueue\0"...,
472) = -1 EAGAIN (Resource temporarily unavailable)
write(35, "HRBT\0\1\0\3d<\230k\0\0\0\0\0\0\1\330\0\0\0\0enqueue\0"...,
472) = -1 EAGAIN (Resource temporarily unavailable)
write(35, "HRBT\0\1\0\3d<\230k\0\0\0\0\0\0\1\330\0\0\0\0enqueue\0"...,
472) = -1 EAGAIN (Resource temporarily unavailable)
File descriptor 35 is the unix socket corresponding to the virtio-serial
port.
I broke in with gdb and got a backtrace showing it was in send_all().
Looking at the implementation of send_all(), the core loop looks like:
while (len > 0) {
ret = write(fd, buf, len);
if (ret < 0) {
if (errno != EINTR && errno != EAGAIN)
return -1;
} else if (ret == 0) {
break;
} else {
buf += ret;
len -= ret;
}
}
So if we get EAGAIN, we'll just immediately retry.
I'm not sure where the unix socket would get opened, but I'm assuming
it's set as non-blocking? And by default
/proc/sys/net/unix/max_dgram_qlen is set to 10.
So if the other end of that unix socket is connected but isn't actually
paying attention to the messages then the first 10 messages will get
buffered but after that we'll end up with qemu spinning forever in a
busy-loop trying to send a message into a full buffer.
This seems less than ideal. Either we should block, or else we should
discard the data. And I don't think discarding the data makes sense.
Chris
