hw/block/dataplane/virtio-blk.c | 67 +++++++++++++++++++-------------- 1 file changed, 38 insertions(+), 29 deletions(-)
The main loop thread can consume 100% CPU when using --device
virtio-blk-pci,iothread=<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 <ldoktor@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
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);
- }
-
- /*
- * 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);
- }
-
qemu_bh_cancel(s->bh);
notify_guest_bh(s); /* final chance to notify guest */
/* Clean up guest notifier (irq) */
k->set_guest_notifiers(qbus->parent, nvqs, false);
- vblk->dataplane_started = false;
s->stopping = false;
}
--
2.40.1
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=<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 <ldoktor@redhat.com> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> > --- > 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
Thank you, Stefan, I tested it with the extended set of tests and it addresses the issue. Regards, Lukáš Tested-by: Lukas Doktor <ldoktor@redhat.com> 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=<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 <ldoktor@redhat.com> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> > --- > 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); > - } > - > - /* > - * 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); > - } > - > qemu_bh_cancel(s->bh); > notify_guest_bh(s); /* final chance to notify guest */ > > /* Clean up guest notifier (irq) */ > k->set_guest_notifiers(qbus->parent, nvqs, false); > > - vblk->dataplane_started = false; > s->stopping = false; > }
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