Re: [Qemu-devel] [Qemu-block] [PATCH] throttle-groups: fix hang when group member leaves

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Wed, Jul 04, 2018 at 03:54:10PM +0100, Stefan Hajnoczi wrote:
> 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>
> 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(+)

Berto is away in July.  I am merging this fix for QEMU 3.0.  If there
are any comments when Berto is back I'll send a follow-up patch.

Applied to my block tree:
https://github.com/stefanha/qemu/commits/block

Stefan

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