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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 31 Oct 2021 10:47:34 -0400

On Thu, Oct 28, 2021 at 02:16:03PM +0800, Xuan Zhuo wrote:
> On Thu, 28 Oct 2021 10:16:10 +0800, Jason Wang <[email-masked]> wrote:
> > On Thu, Oct 28, 2021 at 1:07 AM Michael S. Tsirkin <[email-masked]> 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 <[email-masked]>
> > > > ---
> > > >  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.
> 
> I did a test, the code is as follows:
> 
> +static void desc_cache_put_packed(struct vring_virtqueue *vq,
> +                                  struct vring_packed_desc *desc, int n)
> + {
> +       if (n <= VIRT_QUEUE_CACHE_DESC_NUM) {
> +               kmem_cache_free(vq->desc_cache, desc);
> +       } else {
> +               kfree(desc);
> +       }
> 
> 
> @@ -476,11 +452,14 @@ static struct vring_desc *alloc_indirect_split(struct vring_virtqueue *vq,
>          */
>         gfp &= ~__GFP_HIGHMEM;
> 
> -       desc = kmalloc_array(n, sizeof(struct vring_desc), gfp);
> +       if (total_sg <= VIRT_QUEUE_CACHE_DESC_NUM)
> +               desc = kmem_cache_alloc(vq->desc_cache, gfp);
> +       else
> +               desc = kmalloc_array(total_sg, sizeof(struct vring_desc), gfp);
> +
> 
> 	.......
> 
> +       vq->desc_cache = kmem_cache_create("virio_desc",
> +                                          4 * sizeof(struct vring_desc),
> +                                          0, 0, NULL);
> 
> The effect is not good, basically there is no improvement, using perf top can
> see that the overhead of kmem_cache_alloc/kmem_cache_free is also relatively
> large:
> 
>          26.91%  [kernel]  [k] virtqueue_add
>          15.35%  [kernel]  [k] detach_buf_split
>          14.15%  [kernel]  [k] virtnet_xsk_xmit
>          13.24%  [kernel]  [k] virtqueue_add_outbuf
>        >  9.30%  [kernel]  [k] __slab_free
>        >  3.91%  [kernel]  [k] kmem_cache_alloc
>           2.85%  [kernel]  [k] virtqueue_get_buf_ctx
>        >  2.82%  [kernel]  [k] kmem_cache_free
>           2.54%  [kernel]  [k] memset_erms
>           2.37%  [kernel]  [k] xsk_tx_peek_desc
>           1.20%  [kernel]  [k] virtnet_xsk_run
>           0.81%  [kernel]  [k] vring_map_one_sg
>           0.69%  [kernel]  [k] __free_old_xmit_ptr
>           0.69%  [kernel]  [k] virtqueue_kick_prepare
>           0.43%  [kernel]  [k] sg_init_table
>           0.41%  [kernel]  [k] sg_next
>           0.28%  [kernel]  [k] vring_unmap_one_split
>           0.25%  [kernel]  [k] vring_map_single.constprop.34
>           0.24%  [kernel]  [k] net_rx_action
> 
> Thanks.

How about batching these?  E.g. kmem_cache_alloc_bulk/kmem_cache_free_bulk?

> > Thanks
> > > --
> > > MST