Stefano Garzarella
2022-Jan-28 09:41 UTC
[PATCH v3] vhost: cache avail index in vhost_enable_notify()
In vhost_enable_notify() we enable the notifications and we read the avail index to check if new buffers have become available in the meantime. We do not update the cached avail index value, so when the device will call vhost_get_vq_desc(), it will find the old value in the cache and it will read the avail index again. It would be better to refresh the cache every time we read avail index, so let's change vhost_enable_notify() caching the value in `avail_idx` and compare it with `last_avail_idx` to check if there are new buffers available. We don't expect a significant performance boost because the above path is not very common, indeed vhost_enable_notify() is often called with unlikely(), expecting that avail index has not been updated. We ran virtio-test/vhost-test and noticed minimal improvement as expected. To stress the patch more, we modified vhost_test.ko to call vhost_enable_notify()/vhost_disable_notify() on every cycle when calling vhost_get_vq_desc(); in this case we observed a more evident improvement, with a reduction of the test execution time of about 3.7%. Signed-off-by: Stefano Garzarella <sgarzare at redhat.com> --- v3 - reworded commit description [Stefan] --- drivers/vhost/vhost.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c index 59edb5a1ffe2..07363dff559e 100644 --- a/drivers/vhost/vhost.c +++ b/drivers/vhost/vhost.c @@ -2543,8 +2543,9 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq) &vq->avail->idx, r); return false; } + vq->avail_idx = vhost16_to_cpu(vq, avail_idx); - return vhost16_to_cpu(vq, avail_idx) != vq->avail_idx; + return vq->avail_idx != vq->last_avail_idx; } EXPORT_SYMBOL_GPL(vhost_enable_notify); -- 2.34.1
Stefan Hajnoczi
2022-Feb-02 11:14 UTC
[PATCH v3] vhost: cache avail index in vhost_enable_notify()
On Fri, Jan 28, 2022 at 10:41:29AM +0100, Stefano Garzarella wrote:> In vhost_enable_notify() we enable the notifications and we read > the avail index to check if new buffers have become available in > the meantime. > > We do not update the cached avail index value, so when the device > will call vhost_get_vq_desc(), it will find the old value in the > cache and it will read the avail index again. > > It would be better to refresh the cache every time we read avail > index, so let's change vhost_enable_notify() caching the value in > `avail_idx` and compare it with `last_avail_idx` to check if there > are new buffers available. > > We don't expect a significant performance boost because > the above path is not very common, indeed vhost_enable_notify() > is often called with unlikely(), expecting that avail index has > not been updated. > > We ran virtio-test/vhost-test and noticed minimal improvement as > expected. To stress the patch more, we modified vhost_test.ko to > call vhost_enable_notify()/vhost_disable_notify() on every cycle > when calling vhost_get_vq_desc(); in this case we observed a more > evident improvement, with a reduction of the test execution time > of about 3.7%. > > Signed-off-by: Stefano Garzarella <sgarzare at redhat.com> > --- > v3 > - reworded commit description [Stefan] > --- > drivers/vhost/vhost.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > index 59edb5a1ffe2..07363dff559e 100644 > --- a/drivers/vhost/vhost.c > +++ b/drivers/vhost/vhost.c > @@ -2543,8 +2543,9 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq) > &vq->avail->idx, r); > return false; > } > + vq->avail_idx = vhost16_to_cpu(vq, avail_idx); > > - return vhost16_to_cpu(vq, avail_idx) != vq->avail_idx; > + return vq->avail_idx != vq->last_avail_idx; > } > EXPORT_SYMBOL_GPL(vhost_enable_notify);This changes behavior (fixes a bug?): previously the function returned false when called with avail buffers still pending (vq->last_avail_idx < vq->avail_idx). Now it returns true because we compare against vq->last_avail_idx and I think that's reasonable. Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com> -------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 488 bytes Desc: not available URL: <http://lists.linuxfoundation.org/pipermail/virtualization/attachments/20220202/9f147060/attachment.sig>