netdev - Re: [PATCH 1/3] virtio: cache indirect desc for split

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Date:   Thu, 28 Oct 2021 10:16:10 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        netdev <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH 1/3] virtio: cache indirect desc for split

On Thu, Oct 28, 2021 at 1:07 AM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Wed, Oct 27, 2021 at 02:19:11PM +0800, Xuan Zhuo wrote:
> > In the case of using indirect, indirect desc must be allocated and
> > released each time, which increases a lot of cpu overhead.
> >
> > Here, a cache is added for indirect. If the number of indirect desc to be
> > applied for is less than VIRT_QUEUE_CACHE_DESC_NUM, the desc array with
> > the size of VIRT_QUEUE_CACHE_DESC_NUM is fixed and cached for reuse.
> >
> > Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> > ---
> >  drivers/virtio/virtio.c      |  6 ++++
> >  drivers/virtio/virtio_ring.c | 63 ++++++++++++++++++++++++++++++------
> >  include/linux/virtio.h       | 10 ++++++
> >  3 files changed, 70 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> > index 0a5b54034d4b..04bcb74e5b9a 100644
> > --- a/drivers/virtio/virtio.c
> > +++ b/drivers/virtio/virtio.c
> > @@ -431,6 +431,12 @@ bool is_virtio_device(struct device *dev)
> >  }
> >  EXPORT_SYMBOL_GPL(is_virtio_device);
> >
> > +void virtio_use_desc_cache(struct virtio_device *dev, bool val)
> > +{
> > +     dev->desc_cache = val;
> > +}
> > +EXPORT_SYMBOL_GPL(virtio_use_desc_cache);
> > +
> >  void unregister_virtio_device(struct virtio_device *dev)
> >  {
> >       int index = dev->index; /* save for after device release */
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index dd95dfd85e98..0b9a8544b0e8 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -117,6 +117,10 @@ struct vring_virtqueue {
> >       /* Hint for event idx: already triggered no need to disable. */
> >       bool event_triggered;
> >
> > +     /* Is indirect cache used? */
> > +     bool use_desc_cache;
> > +     void *desc_cache_chain;
> > +
> >       union {
> >               /* Available for split ring */
> >               struct {
> > @@ -423,12 +427,47 @@ static unsigned int vring_unmap_one_split(const struct vring_virtqueue *vq,
> >       return extra[i].next;
> >  }
> >
> > -static struct vring_desc *alloc_indirect_split(struct virtqueue *_vq,
> > +#define VIRT_QUEUE_CACHE_DESC_NUM 4
> > +
> > +static void desc_cache_chain_free_split(void *chain)
> > +{
> > +     struct vring_desc *desc;
> > +
> > +     while (chain) {
> > +             desc = chain;
> > +             chain = (void *)desc->addr;
> > +             kfree(desc);
> > +     }
> > +}
> > +
> > +static void desc_cache_put_split(struct vring_virtqueue *vq,
> > +                              struct vring_desc *desc, int n)
> > +{
> > +     if (vq->use_desc_cache && n <= VIRT_QUEUE_CACHE_DESC_NUM) {
> > +             desc->addr = (u64)vq->desc_cache_chain;
> > +             vq->desc_cache_chain = desc;
> > +     } else {
> > +             kfree(desc);
> > +     }
> > +}
> > +
>
>
> So I have a question here. What happens if we just do:
>
> if (n <= VIRT_QUEUE_CACHE_DESC_NUM) {
>         return kmem_cache_alloc(VIRT_QUEUE_CACHE_DESC_NUM * sizeof desc, gfp)
> } else {
>         return kmalloc_arrat(n, sizeof desc, gfp)
> }
>
> A small change and won't we reap most performance benefits?

Yes, I think we need a benchmark to use private cache to see how much
it can help.

Thanks

>
> --
> MST
>