[pve-devel] [PATCH qemu] Revert "add patch to work around stuck guest IO with iothread and VirtIO block/SCSI"

[Fiona Ebner]

This reverts commit 6b7c1815e1c89cb66ff48fbba6da69fe6d254630.

The attempted fix has been reported to cause high CPU usage after
backup [0]. Not difficult to reproduce and it's iothreads getting
stuck in a loop. Downgrading to pve-qemu-kvm=8.1.2-4 helps which was
also verified by Christian, thanks! The issue this was supposed to fix
is much rarer, so revert for now, while upstream is still working on a
proper fix.

[0]: https://forum.proxmox.com/threads/138140/

Signed-off-by: Fiona Ebner <f.ebner at proxmox.com>
---
 ...work-around-iothread-polling-getting.patch | 66 -------------------
 debian/patches/series                         |  1 -
 2 files changed, 67 deletions(-)
 delete mode 100644 debian/patches/pve/0046-virtio-blk-scsi-work-around-iothread-polling-getting.patch

diff --git a/debian/patches/pve/0046-virtio-blk-scsi-work-around-iothread-polling-getting.patch b/debian/patches/pve/0046-virtio-blk-scsi-work-around-iothread-polling-getting.patch
deleted file mode 100644
index 3ac10a8..0000000
--- a/debian/patches/pve/0046-virtio-blk-scsi-work-around-iothread-polling-getting.patch
+++ /dev/null
@@ -1,66 +0,0 @@
-From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
-From: Fiona Ebner <f.ebner at proxmox.com>
-Date: Tue, 5 Dec 2023 14:05:49 +0100
-Subject: [PATCH] virtio blk/scsi: work around iothread polling getting stuck
- with drain
-
-When using iothread, after commits
-1665d9326f ("virtio-blk: implement BlockDevOps->drained_begin()")
-766aa2de0f ("virtio-scsi: implement BlockDevOps->drained_begin()")
-it can happen that polling gets stuck when draining. This would cause
-IO in the guest to get completely stuck.
-
-A workaround for users is stopping and resuming the vCPUs because that
-would also stop and resume the dataplanes which would kick the host
-notifiers.
-
-This can happen with block jobs like backup and drive mirror as well
-as with hotplug [2].
-
-Reports in the community forum that might be about this issue[0][1]
-and there is also one in the enterprise support channel.
-
-As a workaround in the code, just re-enable notifications and kick the
-virt queue after draining. Draining is already costly and rare, so no
-need to worry about a performance penalty here. This was taken from
-the following comment of a QEMU developer [3] (in my debugging,
-I had already found re-enabling notification to work around the issue,
-but also kicking the queue is more complete).
-
-[0]: https://forum.proxmox.com/threads/137286/
-[1]: https://forum.proxmox.com/threads/137536/
-[2]: https://issues.redhat.com/browse/RHEL-3934
-[3]: https://issues.redhat.com/browse/RHEL-3934?focusedId=23562096&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-23562096
-
-Signed-off-by: Fiona Ebner <f.ebner at proxmox.com>
----
- hw/block/virtio-blk.c | 2 ++
- hw/scsi/virtio-scsi.c | 2 ++
- 2 files changed, 4 insertions(+)
-
-diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
-index 39e7f23fab..22502047d5 100644
---- a/hw/block/virtio-blk.c
-+++ b/hw/block/virtio-blk.c
-@@ -1537,6 +1537,8 @@ static void virtio_blk_drained_end(void *opaque)
-     for (uint16_t i = 0; i < s->conf.num_queues; i++) {
-         VirtQueue *vq = virtio_get_queue(vdev, i);
-         virtio_queue_aio_attach_host_notifier(vq, ctx);
-+        virtio_queue_set_notification(vq, 1);
-+        virtio_queue_notify(vdev, i);
-     }
- }
- 
-diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
-index 45b95ea070..a7bddbf899 100644
---- a/hw/scsi/virtio-scsi.c
-+++ b/hw/scsi/virtio-scsi.c
-@@ -1166,6 +1166,8 @@ static void virtio_scsi_drained_end(SCSIBus *bus)
-     for (uint32_t i = 0; i < total_queues; i++) {
-         VirtQueue *vq = virtio_get_queue(vdev, i);
-         virtio_queue_aio_attach_host_notifier(vq, s->ctx);
-+        virtio_queue_set_notification(vq, 1);
-+        virtio_queue_notify(vdev, i);
-     }
- }
- 
diff --git a/debian/patches/series b/debian/patches/series
index 7dcedcb..b3da8bb 100644
--- a/debian/patches/series
+++ b/debian/patches/series
@@ -60,4 +60,3 @@ pve/0042-Revert-block-rbd-implement-bdrv_co_block_status.patch
 pve/0043-alloc-track-fix-deadlock-during-drop.patch
 pve/0044-migration-for-snapshots-hold-the-BQL-during-setup-ca.patch
 pve/0045-savevm-async-don-t-hold-BQL-during-setup.patch
-pve/0046-virtio-blk-scsi-work-around-iothread-polling-getting.patch
-- 
2.39.2