On Wed, Jul 04, 2018 at 03:54:10PM +0100, Stefan Hajnoczi wrote: Sorry you weren't CCed originally, Berto. This one is for you! :)
> Throttle groups consist of members sharing one throttling state > (including bps/iops limits). Round-robin scheduling is used to ensure > fairness. If a group member already has a timer pending then other > groups members do not schedule their own timers. The next group member > will have its turn when the existing timer expires. > > A hang may occur when a group member leaves while it had a timer > scheduled. Although the code carefully removes the group member from > the round-robin list, it does not schedule the next member. Therefore > remaining members continue to wait for the removed member's timer to > expire. > > This patch schedules the next request if a timer is pending. > Unfortunately the actual bug is a race condition that I've been unable > to capture in a test case. > > Sometimes drive2 hangs when drive1 is removed from the throttling group: > > $ qemu ... -drive > if=none,id=drive1,cache=none,format=qcow2,file=data1.qcow2,iops=100,group=foo > \ > -device virtio-blk-pci,id=virtio-blk-pci0,drive=drive1 \ > -drive > if=none,id=drive2,cache=none,format=qcow2,file=data2.qcow2,iops=10,group=foo \ > -device virtio-blk-pci,id=virtio-blk-pci1,drive=drive2 > (guest-console1)# fio -filename /dev/vda 4k-seq-read.job > (guest-console2)# fio -filename /dev/vdb 4k-seq-read.job > (qmp) {"execute": "block_set_io_throttle", "arguments": {"device": > "drive1","bps": 0,"bps_rd": 0,"bps_wr": 0,"iops": 0,"iops_rd": 0,"iops_wr": > 0}} > > Reported-by: Nini Gu <n...@redhat.com> > RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1535914 > Cc: Alberto Garcia <be...@igalia.com> > Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com> > --- > block/throttle-groups.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/block/throttle-groups.c b/block/throttle-groups.c > index 36cc0430c3..e297b04e17 100644 > --- a/block/throttle-groups.c > +++ b/block/throttle-groups.c > @@ -564,6 +564,10 @@ void throttle_group_unregister_tgm(ThrottleGroupMember > *tgm) > > qemu_mutex_lock(&tg->lock); > for (i = 0; i < 2; i++) { > + if (timer_pending(tgm->throttle_timers.timers[i])) { > + tg->any_timer_armed[i] = false; > + schedule_next_request(tgm, i); > + } > if (tg->tokens[i] == tgm) { > token = throttle_group_next_tgm(tgm); > /* Take care of the case where this is the last tgm in the group > */ > -- > 2.17.1 > >
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