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bh=5+4f1BTBrGPeL+aMgNruwrm/piaTYfPvUr4lyKEqSVQ=; b=fBeIcQP71BklmEr2i4m8Zq6zFL9hvvukpj5Q5X4uJwE7l/5CfJlyUCF8WpMKiTfMhKGpHE ww8tkEYY2381uV+GIY+wbiyp1Hyu0a2qWqlR+wT3qvmuE1krX6BkpwEWDiwmsJp80sRIF7 QirCcgVbU5E1O4sa5x+ZNBv9TZ/GKjc= X-MC-Unique: XIMsJH2aMampqapQmcN98Q-1 From: Stefan Hajnoczi To: qemu-devel@nongnu.org Cc: Kevin Wolf , qemu-block@nongnu.org, Hanna Reitz , Stefan Hajnoczi , =?UTF-8?q?Luk=C3=A1=C5=A1=20Doktor?= Subject: [PATCH] virtio-blk: fix host notifier issues during dataplane start/stop Date: Tue, 4 Jul 2023 17:15:27 +0200 Message-Id: <20230704151527.193586-1-stefanha@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.1 on 10.11.54.1 Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.129.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1688483756584100001 The main loop thread can consume 100% CPU when using --device virtio-blk-pci,iothread=3D. 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: Luk=C3=A1=C5=A1 Doktor Signed-off-by: Stefan Hajnoczi Tested-by: Lukas Doktor --- 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-bl= k.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) =20 memory_region_transaction_commit(); =20 - /* - * These fields are visible to the IOThread so we rely on implicit bar= riers - * in aio_context_acquire() on the write side and aio_notify_accept() = on - * the read side. - */ - s->starting =3D false; - vblk->dataplane_started =3D true; trace_virtio_blk_data_plane_start(s); =20 old_context =3D 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)); } =20 + /* + * 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 =3D false; + vblk->dataplane_started =3D 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 =3D true; s->starting =3D false; - vblk->dataplane_started =3D true; return -ENOSYS; } =20 @@ -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); } =20 + /* + * 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 =3D 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 =3D 0; i < nvqs; i++) { + virtio_bus_cleanup_host_notifier(VIRTIO_BUS(qbus), i); + } + + /* + * Set ->dataplane_started to false before draining so that host notif= iers + * are not detached/attached anymore. + */ + vblk->dataplane_started =3D false; + aio_context_acquire(s->ctx); =20 /* 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) =20 aio_context_release(s->ctx); =20 - /* - * 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 =3D 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 =3D 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 */ =20 /* Clean up guest notifier (irq) */ k->set_guest_notifiers(qbus->parent, nvqs, false); =20 - vblk->dataplane_started =3D false; s->stopping =3D false; } --=20 2.40.1