[Qemu-devel] [PULL for-v3.0 0/1] Block patches

[Qemu-devel] [PULL for-v3.0 0/1] Block patches

[Stefan Hajnoczi]

The following changes since commit ea6abffa8a08d832feb759d359d5b935e3087cf7:

  Update version for v3.0.0-rc1 release (2018-07-17 18:15:19 +0100)

are available in the Git repository at:

  git://github.com/stefanha/qemu.git tags/block-pull-request

for you to fetch changes up to 6fccbb475bc6effc313ee9481726a1748b6dae57:

  throttle-groups: fix hang when group member leaves (2018-07-19 13:08:26 +0100)

----------------------------------------------------------------
Pull request

This fix prevents hangs when a drive leaves a throttling group.

----------------------------------------------------------------

Stefan Hajnoczi (1):
  throttle-groups: fix hang when group member leaves

 block/throttle-groups.c | 4 ++++
 1 file changed, 4 insertions(+)

-- 
2.17.1

[Qemu-devel] [PULL for-v3.0 1/1] throttle-groups: fix hang when group member leaves

[Stefan Hajnoczi]

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 <ngu@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20180704145410.794-1-stefanha@redhat.com
RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1535914
Cc: Alberto Garcia <berto@igalia.com>
Signed-off-by: Stefan Hajnoczi <stefanha@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

Re: [Qemu-devel] [PULL for-v3.0 0/1] Block patches

[Peter Maydell]

On 19 July 2018 at 15:28, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> The following changes since commit ea6abffa8a08d832feb759d359d5b935e3087cf7:
>
>   Update version for v3.0.0-rc1 release (2018-07-17 18:15:19 +0100)
>
> are available in the Git repository at:
>
>   git://github.com/stefanha/qemu.git tags/block-pull-request
>
> for you to fetch changes up to 6fccbb475bc6effc313ee9481726a1748b6dae57:
>
>   throttle-groups: fix hang when group member leaves (2018-07-19 13:08:26 +0100)
>
> ----------------------------------------------------------------
> Pull request
>
> This fix prevents hangs when a drive leaves a throttling group.
>
> ----------------------------------------------------------------
>
> Stefan Hajnoczi (1):
>   throttle-groups: fix hang when group member leaves
>
>  block/throttle-groups.c | 4 ++++
>  1 file changed, 4 insertions(+)

Applied, thanks.

-- PMM