Re: [PATCH] virtio-blk: fix host notifier issues during dataplane start/stop
On Tue, Jul 04, 2023 at 05:15:27PM +0200, Stefan Hajnoczi wrote:
> The main loop thread can consume 100% CPU when using --device
> virtio-blk-pci,iothread=. ppoll() constantly returns but
> reading virtqueue host notifiers fails with EAGAIN. The file descriptors
> are stale and remain registered with the AioContext because of bugs in
> the virtio-blk dataplane start/stop code.
>
> The problem is that the dataplane start/stop code involves drain
> operations, which call virtio_blk_drained_begin() and
> virtio_blk_drained_end() at points where the host notifier is not
> operational:
> - In virtio_blk_data_plane_start(), blk_set_aio_context() drains after
> vblk->dataplane_started has been set to true but the host notifier has
> not been attached yet.
> - In virtio_blk_data_plane_stop(), blk_drain() and blk_set_aio_context()
> drain after the host notifier has already been detached but with
> vblk->dataplane_started still set to true.
>
> I would like to simplify ->ioeventfd_start/stop() to avoid interactions
> with drain entirely, but couldn't find a way to do that. Instead, this
> patch accepts the fragile nature of the code and reorders it so that
> vblk->dataplane_started is false during drain operations. This way the
> virtio_blk_drained_begin() and virtio_blk_drained_end() calls don't
> touch the host notifier. The result is that
> virtio_blk_data_plane_start() and virtio_blk_data_plane_stop() have
> complete control over the host notifier and stale file descriptors are
> no longer left in the AioContext.
>
> This patch fixes the 100% CPU consumption in the main loop thread and
> correctly moves host notifier processing to the IOThread.
>
> Fixes: 1665d9326fd2 ("virtio-blk: implement BlockDevOps->drained_begin()")
> Reported-by: Lukáš Doktor
> Signed-off-by: Stefan Hajnoczi
> ---
> hw/block/dataplane/virtio-blk.c | 67 +++--
> 1 file changed, 38 insertions(+), 29 deletions(-)
Thanks, applied to my block tree:
https://gitlab.com/stefanha/qemu/commits/block
Stefan
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Re: [PATCH] virtio-blk: fix host notifier issues during dataplane start/stop
Thank you, Stefan, I tested it with the extended set of tests and it addresses
the issue.
Regards,
Lukáš
Tested-by: Lukas Doktor
Dne 04. 07. 23 v 17:15 Stefan Hajnoczi napsal(a):
> The main loop thread can consume 100% CPU when using --device
> virtio-blk-pci,iothread=. ppoll() constantly returns but
> reading virtqueue host notifiers fails with EAGAIN. The file descriptors
> are stale and remain registered with the AioContext because of bugs in
> the virtio-blk dataplane start/stop code.
>
> The problem is that the dataplane start/stop code involves drain
> operations, which call virtio_blk_drained_begin() and
> virtio_blk_drained_end() at points where the host notifier is not
> operational:
> - In virtio_blk_data_plane_start(), blk_set_aio_context() drains after
> vblk->dataplane_started has been set to true but the host notifier has
> not been attached yet.
> - In virtio_blk_data_plane_stop(), blk_drain() and blk_set_aio_context()
> drain after the host notifier has already been detached but with
> vblk->dataplane_started still set to true.
>
> I would like to simplify ->ioeventfd_start/stop() to avoid interactions
> with drain entirely, but couldn't find a way to do that. Instead, this
> patch accepts the fragile nature of the code and reorders it so that
> vblk->dataplane_started is false during drain operations. This way the
> virtio_blk_drained_begin() and virtio_blk_drained_end() calls don't
> touch the host notifier. The result is that
> virtio_blk_data_plane_start() and virtio_blk_data_plane_stop() have
> complete control over the host notifier and stale file descriptors are
> no longer left in the AioContext.
>
> This patch fixes the 100% CPU consumption in the main loop thread and
> correctly moves host notifier processing to the IOThread.
>
> Fixes: 1665d9326fd2 ("virtio-blk: implement BlockDevOps->drained_begin()")
> Reported-by: Lukáš Doktor
> Signed-off-by: Stefan Hajnoczi
> ---
> hw/block/dataplane/virtio-blk.c | 67 +++--
> 1 file changed, 38 insertions(+), 29 deletions(-)
>
> diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
> index c227b39408..da36fcfd0b 100644
> --- a/hw/block/dataplane/virtio-blk.c
> +++ b/hw/block/dataplane/virtio-blk.c
> @@ -219,13 +219,6 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev)
>
> memory_region_transaction_commit();
>
> -/*
> - * These fields are visible to the IOThread so we rely on implicit
> barriers
> - * in aio_context_acquire() on the write side and aio_notify_accept() on
> - * the read side.
> - */
> -s->starting = false;
> -vblk->dataplane_started = true;
> trace_virtio_blk_data_plane_start(s);
>
> old_context = blk_get_aio_context(s->conf->conf.blk);
> @@ -244,6 +237,18 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev)
> event_notifier_set(virtio_queue_get_host_notifier(vq));
> }
>
> +/*
> + * These fields must be visible to the IOThread when it processes the
> + * virtqueue, otherwise it will think dataplane has not started yet.
> + *
> + * Make sure ->dataplane_started is false when blk_set_aio_context() is
> + * called above so that draining does not cause the host notifier to be
> + * detached/attached prematurely.
> + */
> +s->starting = false;
> +vblk->dataplane_started = true;
> +smp_wmb(); /* paired with aio_notify_accept() on the read side */
> +
> /* Get this show started by hooking up our callbacks */
> if (!blk_in_drain(s->conf->conf.blk)) {
> aio_context_acquire(s->ctx);
> @@ -273,7 +278,6 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev)
>fail_guest_notifiers:
> vblk->dataplane_disabled = true;
> s->starting = false;
> -vblk->dataplane_started = true;
> return -ENOSYS;
> }
>
> @@ -327,6 +331,32 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
> aio_wait_bh_oneshot(s->ctx, virtio_blk_data_plane_stop_bh, s);
> }
>
> +/*
> + * Batch all the host notifiers in a single transaction to avoid
> + * quadratic time complexity in address_space_update_ioeventfds().
> + */
> +memory_region_transaction_begin();
> +
> +for (i = 0; i < nvqs; i++) {
> +virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), i, false);
> +}
> +
> +/*
> + * The transaction expects the ioeventfds to be open when it
> + * commits. Do it now, before the cleanup loop.
> + */
> +memory_region_transaction_commit();
> +
> +for (i = 0; i < nvqs; i++) {
> +virtio_bus_cleanup_host_notifier(VIRTIO_BUS(qbus), i);
> +}
> +
> +/*
> + * Set ->dataplane_started to false before draining so that host
> notifiers
> + * are not detached/attached anymore.
> + */
> +vblk->dataplane_started = false;
> +
> aio_context_acquire(s->ctx);
>
> /* Wait for virtio_blk_dma_restart_bh() and in flight I/O to complete */
> @@ -340,32 +370,11
[PATCH] virtio-blk: fix host notifier issues during dataplane start/stop
The main loop thread can consume 100% CPU when using --device
virtio-blk-pci,iothread=. ppoll() constantly returns but
reading virtqueue host notifiers fails with EAGAIN. The file descriptors
are stale and remain registered with the AioContext because of bugs in
the virtio-blk dataplane start/stop code.
The problem is that the dataplane start/stop code involves drain
operations, which call virtio_blk_drained_begin() and
virtio_blk_drained_end() at points where the host notifier is not
operational:
- In virtio_blk_data_plane_start(), blk_set_aio_context() drains after
vblk->dataplane_started has been set to true but the host notifier has
not been attached yet.
- In virtio_blk_data_plane_stop(), blk_drain() and blk_set_aio_context()
drain after the host notifier has already been detached but with
vblk->dataplane_started still set to true.
I would like to simplify ->ioeventfd_start/stop() to avoid interactions
with drain entirely, but couldn't find a way to do that. Instead, this
patch accepts the fragile nature of the code and reorders it so that
vblk->dataplane_started is false during drain operations. This way the
virtio_blk_drained_begin() and virtio_blk_drained_end() calls don't
touch the host notifier. The result is that
virtio_blk_data_plane_start() and virtio_blk_data_plane_stop() have
complete control over the host notifier and stale file descriptors are
no longer left in the AioContext.
This patch fixes the 100% CPU consumption in the main loop thread and
correctly moves host notifier processing to the IOThread.
Fixes: 1665d9326fd2 ("virtio-blk: implement BlockDevOps->drained_begin()")
Reported-by: Lukáš Doktor
Signed-off-by: Stefan Hajnoczi
---
hw/block/dataplane/virtio-blk.c | 67 +++--
1 file changed, 38 insertions(+), 29 deletions(-)
diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
index c227b39408..da36fcfd0b 100644
--- a/hw/block/dataplane/virtio-blk.c
+++ b/hw/block/dataplane/virtio-blk.c
@@ -219,13 +219,6 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev)
memory_region_transaction_commit();
-/*
- * These fields are visible to the IOThread so we rely on implicit barriers
- * in aio_context_acquire() on the write side and aio_notify_accept() on
- * the read side.
- */
-s->starting = false;
-vblk->dataplane_started = true;
trace_virtio_blk_data_plane_start(s);
old_context = blk_get_aio_context(s->conf->conf.blk);
@@ -244,6 +237,18 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev)
event_notifier_set(virtio_queue_get_host_notifier(vq));
}
+/*
+ * These fields must be visible to the IOThread when it processes the
+ * virtqueue, otherwise it will think dataplane has not started yet.
+ *
+ * Make sure ->dataplane_started is false when blk_set_aio_context() is
+ * called above so that draining does not cause the host notifier to be
+ * detached/attached prematurely.
+ */
+s->starting = false;
+vblk->dataplane_started = true;
+smp_wmb(); /* paired with aio_notify_accept() on the read side */
+
/* Get this show started by hooking up our callbacks */
if (!blk_in_drain(s->conf->conf.blk)) {
aio_context_acquire(s->ctx);
@@ -273,7 +278,6 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev)
fail_guest_notifiers:
vblk->dataplane_disabled = true;
s->starting = false;
-vblk->dataplane_started = true;
return -ENOSYS;
}
@@ -327,6 +331,32 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
aio_wait_bh_oneshot(s->ctx, virtio_blk_data_plane_stop_bh, s);
}
+/*
+ * Batch all the host notifiers in a single transaction to avoid
+ * quadratic time complexity in address_space_update_ioeventfds().
+ */
+memory_region_transaction_begin();
+
+for (i = 0; i < nvqs; i++) {
+virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), i, false);
+}
+
+/*
+ * The transaction expects the ioeventfds to be open when it
+ * commits. Do it now, before the cleanup loop.
+ */
+memory_region_transaction_commit();
+
+for (i = 0; i < nvqs; i++) {
+virtio_bus_cleanup_host_notifier(VIRTIO_BUS(qbus), i);
+}
+
+/*
+ * Set ->dataplane_started to false before draining so that host notifiers
+ * are not detached/attached anymore.
+ */
+vblk->dataplane_started = false;
+
aio_context_acquire(s->ctx);
/* Wait for virtio_blk_dma_restart_bh() and in flight I/O to complete */
@@ -340,32 +370,11 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
aio_context_release(s->ctx);
-/*
- * Batch all the host notifiers in a single transaction to avoid
- * quadratic time complexity in address_space_update_ioeventfds().
- */
-memory_region_transaction_begin();
-
-for (i = 0; i < nvqs; i++) {
-virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), i, false);
-}
-
-/*
- *
