75dcb4d790
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:
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| .. | ||
| meson.build | ||
| trace-events | ||
| trace.h | ||
| virtio-blk.c | ||
| virtio-blk.h | ||
| xen-block.c | ||
| xen-block.h | ||