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[Qemu-commits] [qemu/qemu] 6fccbb: throttle-groups: fix hang when group

From: GitHub
Subject: [Qemu-commits] [qemu/qemu] 6fccbb: throttle-groups: fix hang when group member leaves
Date: Thu, 19 Jul 2018 08:26:12 -0700

  Branch: refs/heads/master
  Home:   https://github.com/qemu/qemu
  Commit: 6fccbb475bc6effc313ee9481726a1748b6dae57
  Author: Stefan Hajnoczi <address@hidden>
  Date:   2018-07-19 (Thu, 19 Jul 2018)

  Changed paths:
    M block/throttle-groups.c

  Log Message:
  throttle-groups: fix hang when group member leaves

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

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 \
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 <address@hidden>
Signed-off-by: Stefan Hajnoczi <address@hidden>
Message-id: address@hidden
RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1535914
Cc: Alberto Garcia <address@hidden>
Signed-off-by: Stefan Hajnoczi <address@hidden>

  Commit: e1ea55668ffe6ce558a063f3a9621b761738e1f2
  Author: Peter Maydell <address@hidden>
  Date:   2018-07-19 (Thu, 19 Jul 2018)

  Changed paths:
    M block/throttle-groups.c

  Log Message:
  Merge remote-tracking branch 'remotes/stefanha/tags/block-pull-request' into 

Pull request

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

# gpg: Signature made Thu 19 Jul 2018 15:28:37 BST
# gpg:                using RSA key 9CA4ABB381AB73C8
# gpg: Good signature from "Stefan Hajnoczi <address@hidden>"
# gpg:                 aka "Stefan Hajnoczi <address@hidden>"
# Primary key fingerprint: 8695 A8BF D3F9 7CDA AC35  775A 9CA4 ABB3 81AB 73C8

* remotes/stefanha/tags/block-pull-request:
  throttle-groups: fix hang when group member leaves

Signed-off-by: Peter Maydell <address@hidden>

Compare: https://github.com/qemu/qemu/compare/771eab934943...e1ea55668ffe
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