From: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Willem de Bruijn <willemb@google.com>,
netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org,
Jakub Kicinski <kuba@kernel.org>, Wei Wang <weiwan@google.com>,
David Miller <davem@davemloft.net>
Subject: Re: [PATCH v3 3/4] virtio: fix up virtio_disable_cb
Date: Thu, 30 Mar 2023 14:54:21 +0800 [thread overview]
Message-ID: <1680159261.1588428-1-xuanzhuo@linux.alibaba.com> (raw)
In-Reply-To: <20230330024220-mutt-send-email-mst@kernel.org>
On Thu, 30 Mar 2023 02:44:44 -0400, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> On Thu, Mar 30, 2023 at 02:07:37PM +0800, Xuan Zhuo wrote:
> > On Wed, 26 May 2021 04:24:40 -0400, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > > virtio_disable_cb is currently a nop for split ring with event index.
> > > This is because it used to be always called from a callback when we know
> > > device won't trigger more events until we update the index. However,
> > > now that we run with interrupts enabled a lot we also poll without a
> > > callback so that is different: disabling callbacks will help reduce the
> > > number of spurious interrupts.
> > > Further, if using event index with a packed ring, and if being called
> > > from a callback, we actually do disable interrupts which is unnecessary.
> > >
> > > Fix both issues by tracking whenever we get a callback. If that is
> > > the case disabling interrupts with event index can be a nop.
> > > If not the case disable interrupts. Note: with a split ring
> > > there's no explicit "no interrupts" value. For now we write
> > > a fixed value so our chance of triggering an interupt
> > > is 1/ring size. It's probably better to write something
> > > related to the last used index there to reduce the chance
> > > even further. For now I'm keeping it simple.
> >
> >
> > Don't understand, is this patch necessary? For this patch set, we can do without
> > this patch.
> >
> > So doest this patch optimize virtqueue_disable_cb() by reducing a modification of
> > vring_used_event(&vq-> split.vring)?
> >
> > Or I miss something.
> >
> > Thanks.
>
> Before this patch virtqueue_disable_cb did nothing at all
> for the common case of event index enabled, so
> calling it from virtio net would not help matters.
I agree with these codes:
- if (!vq->event)
+ if (vq->event)
+ /* TODO: this is a hack. Figure out a cleaner value to write. */
+ vring_used_event(&vq->split.vring) = 0x0;
+ else
I just don't understand event_triggered.
>
> But the patch is from 2021, isn't it a bit too late to argue?
> If you have a cleanup or an optimization in mind, please
> post a patch.
Sorry, I just have some problems, I don't oppose it. At least it can reduce the
modification of vring_used_event(&vq->split.vring). I think it is also beneficial.
Thanks very much.
>
> > >
> > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > > ---
> > > drivers/virtio/virtio_ring.c | 26 +++++++++++++++++++++++++-
> > > 1 file changed, 25 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > index 71e16b53e9c1..88f0b16b11b8 100644
> > > --- a/drivers/virtio/virtio_ring.c
> > > +++ b/drivers/virtio/virtio_ring.c
> > > @@ -113,6 +113,9 @@ struct vring_virtqueue {
> > > /* Last used index we've seen. */
> > > u16 last_used_idx;
> > >
> > > + /* Hint for event idx: already triggered no need to disable. */
> > > + bool event_triggered;
> > > +
> > > union {
> > > /* Available for split ring */
> > > struct {
> > > @@ -739,7 +742,10 @@ static void virtqueue_disable_cb_split(struct virtqueue *_vq)
> > >
> > > if (!(vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
> > > vq->split.avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
> > > - if (!vq->event)
> > > + if (vq->event)
> > > + /* TODO: this is a hack. Figure out a cleaner value to write. */
> > > + vring_used_event(&vq->split.vring) = 0x0;
> > > + else
> > > vq->split.vring.avail->flags =
> > > cpu_to_virtio16(_vq->vdev,
> > > vq->split.avail_flags_shadow);
> > > @@ -1605,6 +1611,7 @@ static struct virtqueue *vring_create_virtqueue_packed(
> > > vq->weak_barriers = weak_barriers;
> > > vq->broken = false;
> > > vq->last_used_idx = 0;
> > > + vq->event_triggered = false;
> > > vq->num_added = 0;
> > > vq->packed_ring = true;
> > > vq->use_dma_api = vring_use_dma_api(vdev);
> > > @@ -1919,6 +1926,12 @@ void virtqueue_disable_cb(struct virtqueue *_vq)
> > > {
> > > struct vring_virtqueue *vq = to_vvq(_vq);
> > >
> > > + /* If device triggered an event already it won't trigger one again:
> > > + * no need to disable.
> > > + */
> > > + if (vq->event_triggered)
> > > + return;
> > > +
> > > if (vq->packed_ring)
> > > virtqueue_disable_cb_packed(_vq);
> > > else
> > > @@ -1942,6 +1955,9 @@ unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq)
> > > {
> > > struct vring_virtqueue *vq = to_vvq(_vq);
> > >
> > > + if (vq->event_triggered)
> > > + vq->event_triggered = false;
> > > +
> > > return vq->packed_ring ? virtqueue_enable_cb_prepare_packed(_vq) :
> > > virtqueue_enable_cb_prepare_split(_vq);
> > > }
> > > @@ -2005,6 +2021,9 @@ bool virtqueue_enable_cb_delayed(struct virtqueue *_vq)
> > > {
> > > struct vring_virtqueue *vq = to_vvq(_vq);
> > >
> > > + if (vq->event_triggered)
> > > + vq->event_triggered = false;
> > > +
> > > return vq->packed_ring ? virtqueue_enable_cb_delayed_packed(_vq) :
> > > virtqueue_enable_cb_delayed_split(_vq);
> > > }
> > > @@ -2044,6 +2063,10 @@ irqreturn_t vring_interrupt(int irq, void *_vq)
> > > if (unlikely(vq->broken))
> > > return IRQ_HANDLED;
> > >
> > > + /* Just a hint for performance: so it's ok that this can be racy! */
> > > + if (vq->event)
> > > + vq->event_triggered = true;
> > > +
> > > pr_debug("virtqueue callback for %p (%p)\n", vq, vq->vq.callback);
> > > if (vq->vq.callback)
> > > vq->vq.callback(&vq->vq);
> > > @@ -2083,6 +2106,7 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
> > > vq->weak_barriers = weak_barriers;
> > > vq->broken = false;
> > > vq->last_used_idx = 0;
> > > + vq->event_triggered = false;
> > > vq->num_added = 0;
> > > vq->use_dma_api = vring_use_dma_api(vdev);
> > > #ifdef DEBUG
> > > --
> > > MST
> > >
> > > _______________________________________________
> > > Virtualization mailing list
> > > Virtualization@lists.linux-foundation.org
> > > https://lists.linuxfoundation.org/mailman/listinfo/virtualization
>
_______________________________________________
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Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization
next prev parent reply other threads:[~2023-03-30 6:59 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-26 8:24 [PATCH v3 0/4] virtio net: spurious interrupt related fixes Michael S. Tsirkin
2021-05-26 8:24 ` [PATCH v3 1/4] virtio_net: move tx vq operation under tx queue lock Michael S. Tsirkin
2021-05-27 3:41 ` Jason Wang
2021-05-28 22:25 ` Willem de Bruijn
2021-06-09 22:03 ` Michael S. Tsirkin
2021-05-26 8:24 ` [PATCH v3 2/4] virtio_net: move txq wakeups under tx q lock Michael S. Tsirkin
2021-05-27 3:48 ` Jason Wang
2021-05-26 8:24 ` [PATCH v3 3/4] virtio: fix up virtio_disable_cb Michael S. Tsirkin
2021-05-27 4:01 ` Jason Wang
2023-03-30 6:07 ` Xuan Zhuo
2023-03-30 6:44 ` Michael S. Tsirkin
2023-03-30 6:54 ` Xuan Zhuo [this message]
2023-03-30 14:04 ` Michael S. Tsirkin
2023-03-31 3:38 ` Xuan Zhuo
2021-05-26 8:24 ` [PATCH v3 4/4] virtio_net: disable cb aggressively Michael S. Tsirkin
2021-05-26 15:15 ` Eric Dumazet
2021-05-26 21:22 ` Willem de Bruijn
2021-05-27 4:09 ` Jason Wang
2023-01-16 13:41 ` Laurent Vivier
2023-01-17 3:48 ` Jason Wang
2021-05-26 15:34 ` [PATCH v3 0/4] virtio net: spurious interrupt related fixes Willem de Bruijn
2021-06-01 2:53 ` Willem de Bruijn
2021-06-09 21:36 ` Willem de Bruijn
2021-06-09 22:59 ` Willem de Bruijn
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