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From 542fe943f769002a9826f057fcf32acdb1cdc41e Mon Sep 17 00:00:00 2001
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From: Stefan Hajnoczi <stefanha@redhat.com>
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Date: Mon, 23 Jul 2018 16:36:29 +0200
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Subject: [PATCH 89/89] throttle-groups: fix hang when group member leaves
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RH-Author: Stefan Hajnoczi <stefanha@redhat.com>
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Message-id: <20180723163629.7600-2-stefanha@redhat.com>
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Patchwork-id: 81469
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O-Subject: [RHEL-7.6 qemu-kvm-rhev PATCH 1/1] throttle-groups: fix hang when group member leaves
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Bugzilla: 1535914
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RH-Acked-by: Fam Zheng <famz@redhat.com>
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RH-Acked-by: Laurent Vivier <lvivier@redhat.com>
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RH-Acked-by: Miroslav Rezanina <mrezanin@redhat.com>
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Throttle groups consist of members sharing one throttling state
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(including bps/iops limits). Round-robin scheduling is used to ensure
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fairness. If a group member already has a timer pending then other
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groups members do not schedule their own timers. The next group member
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will have its turn when the existing timer expires.
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A hang may occur when a group member leaves while it had a timer
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scheduled. Although the code carefully removes the group member from
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the round-robin list, it does not schedule the next member. Therefore
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remaining members continue to wait for the removed member's timer to
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expire.
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This patch schedules the next request if a timer is pending.
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Unfortunately the actual bug is a race condition that I've been unable
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to capture in a test case.
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Sometimes drive2 hangs when drive1 is removed from the throttling group:
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$ qemu ... -drive if=none,id=drive1,cache=none,format=qcow2,file=data1.qcow2,iops=100,group=foo \
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-device virtio-blk-pci,id=virtio-blk-pci0,drive=drive1 \
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-drive if=none,id=drive2,cache=none,format=qcow2,file=data2.qcow2,iops=10,group=foo \
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-device virtio-blk-pci,id=virtio-blk-pci1,drive=drive2
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(guest-console1)# fio -filename /dev/vda 4k-seq-read.job
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(guest-console2)# fio -filename /dev/vdb 4k-seq-read.job
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(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}}
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Reported-by: Nini Gu <ngu@redhat.com>
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Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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Message-id: 20180704145410.794-1-stefanha@redhat.com
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Cc: Alberto Garcia <berto@igalia.com>
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Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
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---
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block/throttle-groups.c | 4 ++++
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1 file changed, 4 insertions(+)
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diff --git a/block/throttle-groups.c b/block/throttle-groups.c
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index 36cc043..e297b04 100644
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--- a/block/throttle-groups.c
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+++ b/block/throttle-groups.c
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@@ -564,6 +564,10 @@ void throttle_group_unregister_tgm(ThrottleGroupMember *tgm)
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qemu_mutex_lock(&tg->lock);
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for (i = 0; i < 2; i++) {
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+ if (timer_pending(tgm->throttle_timers.timers[i])) {
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+ tg->any_timer_armed[i] = false;
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+ schedule_next_request(tgm, i);
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+ }
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if (tg->tokens[i] == tgm) {
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token = throttle_group_next_tgm(tgm);
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/* Take care of the case where this is the last tgm in the group */
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--
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1.8.3.1
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