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
