Am 22.01.24 um 18:52 schrieb Hanna Czenczek: > On 22.01.24 18:41, Hanna Czenczek wrote: >> On 05.01.24 15:30, Fiona Ebner wrote: >>> Am 05.01.24 um 14:43 schrieb Fiona Ebner: >>>> Am 03.01.24 um 14:35 schrieb Paolo Bonzini: >>>>> On 1/3/24 12:40, Fiona Ebner wrote: >>>>>> I'm happy to report that I cannot reproduce the CPU-usage-spike issue >>>>>> with the patch, but I did run into an assertion failure when >>>>>> trying to >>>>>> verify that it fixes my original stuck-guest-IO issue. See below >>>>>> for the >>>>>> backtrace [0]. Hanna wrote in >>>>>> https://issues.redhat.com/browse/RHEL-3934 >>>>>> >>>>>>> I think it’s sufficient to simply call virtio_queue_notify_vq(vq) >>>>>>> after the virtio_queue_aio_attach_host_notifier(vq, ctx) call, >>>>>>> because >>>>>>> both virtio-scsi’s and virtio-blk’s .handle_output() implementations >>>>>>> acquire the device’s context, so this should be directly callable >>>>>>> from >>>>>>> any context. >>>>>> I guess this is not true anymore now that the AioContext locking was >>>>>> removed? >>>>> Good point and, in fact, even before it was much safer to use >>>>> virtio_queue_notify() instead. Not only does it use the event >>>>> notifier >>>>> handler, but it also calls it in the right thread/AioContext just by >>>>> doing event_notifier_set(). >>>>> >>>> But with virtio_queue_notify() using the event notifier, the >>>> CPU-usage-spike issue is present: >>>> >>>>>> Back to the CPU-usage-spike issue: I experimented around and it >>>>>> doesn't >>>>>> seem to matter whether I notify the virt queue before or after >>>>>> attaching >>>>>> the notifiers. But there's another functional difference. My patch >>>>>> called virtio_queue_notify() which contains this block: >>>>>> >>>>>>> if (vq->host_notifier_enabled) { >>>>>>> event_notifier_set(&vq->host_notifier); >>>>>>> } else if (vq->handle_output) { >>>>>>> vq->handle_output(vdev, vq); >>>>>> In my testing, the first branch was taken, calling >>>>>> event_notifier_set(). >>>>>> Hanna's patch uses virtio_queue_notify_vq() and there, >>>>>> vq->handle_output() will be called. That seems to be the relevant >>>>>> difference regarding the CPU-usage-spike issue. >>>> I should mention that this is with a VirtIO SCSI disk. I also attempted >>>> to reproduce the CPU-usage-spike issue with a VirtIO block disk, but >>>> didn't manage yet. >>>> >>>> What I noticed is that in virtio_queue_host_notifier_aio_poll(), one of >>>> the queues (but only one) will always show as nonempty. And then, >>>> run_poll_handlers_once() will always detect progress which explains the >>>> CPU usage. >>>> >>>> The following shows >>>> 1. vq address >>>> 2. number of times vq was passed to >>>> virtio_queue_host_notifier_aio_poll() >>>> 3. number of times the result of virtio_queue_host_notifier_aio_poll() >>>> was true for the vq >>>> >>>>> 0x555fd93f9c80 17162000 0 >>>>> 0x555fd93f9e48 17162000 6 >>>>> 0x555fd93f9ee0 17162000 0 >>>>> 0x555fd93f9d18 17162000 17162000 >>>>> 0x555fd93f9db0 17162000 0 >>>>> 0x555fd93f9f78 17162000 0 >>>> And for the problematic one, the reason it is seen as nonempty is: >>>> >>>>> 0x555fd93f9d18 shadow_avail_idx 8 last_avail_idx 0 >>> vring_avail_idx(vq) also gives 8 here. This is the vs->event_vq and >>> s->events_dropped is false in my testing, so >>> virtio_scsi_handle_event_vq() doesn't do anything. >>> >>>> Those values stay like this while the call counts above increase. >>>> >>>> So something going wrong with the indices when the event notifier is >>>> set >>>> from QEMU side (in the main thread)? >>>> >>>> The guest is Debian 12 with a 6.1 kernel. >> >> So, trying to figure out a new RFC version: >> >> About the stack trace you, Fiona, posted: As far as I understand, >> that happens because virtio_blk_drained_end() calling >> virtio_queue_notify_vq() wasn’t safe after all, and instead we need to >> use virtio_queue_notify(). Right? >>
AFAICT, yes. In particular, after 4f36b13847 ("scsi: remove AioContext locking"), the AioContext is not acquired by virtio_scsi_handle_{cmd,ctrl,event} anymore. >> However, you say using virtio_queue_notify() instead causes busy loops >> of doing nothing in virtio-scsi (what you describe above). I mean, >> better than a crash, but, you know. :) Yes, that happens for me when using virtio_queue_notify(), i.e. > diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c > index 690aceec45..8cdf04ac2d 100644 > --- a/hw/scsi/virtio-scsi.c > +++ b/hw/scsi/virtio-scsi.c > @@ -1166,7 +1166,11 @@ static void virtio_scsi_drained_end(SCSIBus *bus) > > for (uint32_t i = 0; i < total_queues; i++) { > VirtQueue *vq = virtio_get_queue(vdev, i); > + if (!virtio_queue_get_notification(vq)) { > + virtio_queue_set_notification(vq, true); > + } > virtio_queue_aio_attach_host_notifier(vq, s->ctx); > + virtio_queue_notify(vdev, i); > } > } > >> >> I don’t know have any prior knowledge about the event handling, >> unfortunately. The fact that 8 buffers are available but we don’t use >> any sounds OK to me; as far as I understand, we only use those buffers >> if we have any events to push into them, so as long as we don’t, we >> won’t. Question is, should we not have its poll handler return false >> if we don’t have any events (i.e. events_dropped is false)? Would >> that solve it? > > Or actually, maybe we could just skip the virtio_queue_notify() call for > the event vq? I.e. have it be `if (vq != > VIRTIO_SCSI_COMMON(s)->event_vq) { virtio_queue_notify(vdev, i); }`? That seems to avoid the CPU-usage-spike issue :) > I wouldn’t like that very much, (a) because this would make it slightly > cumbersome to put that into virtio_queue_aio_attach_host_notifier*(), > and (b) in case that does fix it, I do kind of feel like the real > problem is that we use virtio_queue_host_notifier_aio_poll() for the > event vq, which tells the polling code to poll whenever the vq is > non-empty, but we (AFAIU) expect the event vq to be non-empty all the time. > AFAIU, (at least in my testing) it's only non-empty after it was notified via virtio_scsi_drained_end() once. But it's hard to tell, because it seems that the poll callback is only called after the first drain. I noticed poll_set_started() is not called, because ctx->fdmon_ops->need_wait(ctx) was true, i.e. ctx->poll_disable_cnt was positive (I'm using fdmon_poll). I then found this is because of the notifier for the event vq, being attached with > virtio_queue_aio_attach_host_notifier_no_poll(vs->event_vq, s->ctx); in virtio_scsi_dataplane_start(). But in virtio_scsi_drained_end() it is attached with virtio_queue_aio_attach_host_notifier() instead of the _no_poll() variant. So that might be the actual issue here? >From a quick test, I cannot see the CPU-usage-spike issue with the following either: > diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c > index 690aceec45..ba1ab8e410 100644 > --- a/hw/scsi/virtio-scsi.c > +++ b/hw/scsi/virtio-scsi.c > @@ -1166,7 +1166,15 @@ static void virtio_scsi_drained_end(SCSIBus *bus) > > for (uint32_t i = 0; i < total_queues; i++) { > VirtQueue *vq = virtio_get_queue(vdev, i); > - virtio_queue_aio_attach_host_notifier(vq, s->ctx); > + if (!virtio_queue_get_notification(vq)) { > + virtio_queue_set_notification(vq, true); > + } > + if (vq == VIRTIO_SCSI_COMMON(s)->event_vq) { > + virtio_queue_aio_attach_host_notifier_no_poll(vq, s->ctx); > + } else { > + virtio_queue_aio_attach_host_notifier(vq, s->ctx); > + } > + virtio_queue_notify(vdev, i); > } > } Best Regards, Fiona