4ff04bdfa5
This essentially repeats commit6b7c181("add patch to work around stuck guest IO with iothread and VirtIO block/SCSI") with an added fix for the SCSI event virtqueue, which requires special handling. This is to avoid the issue [3] that made the revert2a49e66("Revert "add patch to work around stuck guest IO with iothread and VirtIO block/SCSI"") necessary the first time around. 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. Take special care to attach the SCSI event virtqueue host notifier with the _no_poll() variant like in virtio_scsi_dataplane_start(). This avoids the issue from the first attempted fix where the iothread would suddenly loop with 100% CPU usage whenever some guest IO came in [3]. This is necessary because of commit 38738f7dbb ("virtio-scsi: don't waste CPU polling the event virtqueue"). See [4] for the relevant discussion. [0]: https://forum.proxmox.com/threads/137286/ [1]: https://forum.proxmox.com/threads/137536/ [2]: https://issues.redhat.com/browse/RHEL-3934 [3]: https://forum.proxmox.com/threads/138140/ [4]: https://lore.kernel.org/qemu-devel/bfc7b20c-2144-46e9-acbc-e726276c5a31@proxmox.com/ Link: https://lore.kernel.org/qemu-devel/20240202153158.788922-1-hreitz@redhat.com/ Originally-by: Fiona Ebner <f.ebner@proxmox.com> [ TL: Update to v2 and rebased patch series handling to v8.1.5 ] Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
66 lines
2.8 KiB
Diff
66 lines
2.8 KiB
Diff
From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Hanna Czenczek <hreitz@redhat.com>
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Date: Fri, 2 Feb 2024 16:31:56 +0100
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Subject: [PATCH] virtio-scsi: Attach event vq notifier with no_poll
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As of commit 38738f7dbbda90fbc161757b7f4be35b52205552 ("virtio-scsi:
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don't waste CPU polling the event virtqueue"), we only attach an io_read
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notifier for the virtio-scsi event virtqueue instead, and no polling
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notifiers. During operation, the event virtqueue is typically
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non-empty, but none of the buffers are intended to be used immediately.
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Instead, they only get used when certain events occur. Therefore, it
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makes no sense to continuously poll it when non-empty, because it is
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supposed to be and stay non-empty.
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We do this by using virtio_queue_aio_attach_host_notifier_no_poll()
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instead of virtio_queue_aio_attach_host_notifier() for the event
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virtqueue.
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Commit 766aa2de0f29b657148e04599320d771c36fd126 ("virtio-scsi: implement
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BlockDevOps->drained_begin()") however has virtio_scsi_drained_end() use
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virtio_queue_aio_attach_host_notifier() for all virtqueues, including
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the event virtqueue. This can lead to it being polled again, undoing
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the benefit of commit 38738f7dbbda90fbc161757b7f4be35b52205552.
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Fix it by using virtio_queue_aio_attach_host_notifier_no_poll() for the
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event virtqueue.
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("virtio-scsi: implement BlockDevOps->drained_begin()")
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Reported-by: Fiona Ebner <f.ebner@proxmox.com>
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Fixes: 766aa2de0f29b657148e04599320d771c36fd126
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Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
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Tested-by: Fiona Ebner <f.ebner@proxmox.com>
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Reviewed-by: Fiona Ebner <f.ebner@proxmox.com>
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Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
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Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
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---
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hw/scsi/virtio-scsi.c | 7 ++++++-
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1 file changed, 6 insertions(+), 1 deletion(-)
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diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
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index 45b95ea070..ad24a882fd 100644
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--- a/hw/scsi/virtio-scsi.c
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+++ b/hw/scsi/virtio-scsi.c
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@@ -1148,6 +1148,7 @@ static void virtio_scsi_drained_begin(SCSIBus *bus)
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static void virtio_scsi_drained_end(SCSIBus *bus)
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{
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VirtIOSCSI *s = container_of(bus, VirtIOSCSI, bus);
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+ VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(s);
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VirtIODevice *vdev = VIRTIO_DEVICE(s);
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uint32_t total_queues = VIRTIO_SCSI_VQ_NUM_FIXED +
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s->parent_obj.conf.num_queues;
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@@ -1165,7 +1166,11 @@ static void virtio_scsi_drained_end(SCSIBus *bus)
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for (uint32_t i = 0; i < total_queues; i++) {
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VirtQueue *vq = virtio_get_queue(vdev, i);
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- virtio_queue_aio_attach_host_notifier(vq, s->ctx);
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+ if (vq == vs->event_vq) {
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+ virtio_queue_aio_attach_host_notifier_no_poll(vq, s->ctx);
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+ } else {
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+ virtio_queue_aio_attach_host_notifier(vq, s->ctx);
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+ }
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}
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}
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