Subject: Re: [PATCH v4 0/3] virtio support cache indirect desc

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 8 Nov 2021 08:49:27 -0500

On Mon, Nov 08, 2021 at 07:49:48PM +0800, Xuan Zhuo wrote:
> If the VIRTIO_RING_F_INDIRECT_DESC negotiation succeeds, and the number
> of sgs used for sending packets is greater than 1. We must constantly
> call __kmalloc/kfree to allocate/release desc.
> 
> In the case of extremely fast package delivery, the overhead cannot be
> ignored:
> 
>   27.46%  [kernel]  [k] virtqueue_add
>   16.66%  [kernel]  [k] detach_buf_split
>   16.51%  [kernel]  [k] virtnet_xsk_xmit
>   14.04%  [kernel]  [k] virtqueue_add_outbuf
>    5.18%  [kernel]  [k] __kmalloc
>    4.08%  [kernel]  [k] kfree
>    2.80%  [kernel]  [k] virtqueue_get_buf_ctx
>    2.22%  [kernel]  [k] xsk_tx_peek_desc
>    2.08%  [kernel]  [k] memset_erms
>    0.83%  [kernel]  [k] virtqueue_kick_prepare
>    0.76%  [kernel]  [k] virtnet_xsk_run
>    0.62%  [kernel]  [k] __free_old_xmit_ptr
>    0.60%  [kernel]  [k] vring_map_one_sg
>    0.53%  [kernel]  [k] native_apic_mem_write
>    0.46%  [kernel]  [k] sg_next
>    0.43%  [kernel]  [k] sg_init_table
>    0.41%  [kernel]  [k] kmalloc_slab
> 
> This patch adds a cache function to virtio to cache these allocated indirect
> desc instead of constantly allocating and releasing desc.

Hmm a bunch of comments got ignored. See e.g.
https://lore.kernel.org/r/20211027043851-mutt-send-email-mst%40kernel.org
if they aren't relevant add code comments or commit log text explaining the
design choice please.

> v4:
>     1. Only allow desc cache when VIRTIO_RING_F_INDIRECT_DESC negotiation is successful
>     2. The desc cache threshold can be set for each virtqueue
> 
> v3:
>   pre-allocate per buffer indirect descriptors array
> 
> v2:
>   use struct list_head to cache the desc
> 
> Xuan Zhuo (3):
>   virtio: cache indirect desc for split
>   virtio: cache indirect desc for packed
>   virtio-net: enable virtio desc cache
> 
>  drivers/net/virtio_net.c     |  12 ++-
>  drivers/virtio/virtio_ring.c | 152 +++++++++++++++++++++++++++++++----
>  include/linux/virtio.h       |  17 ++++
>  3 files changed, 163 insertions(+), 18 deletions(-)
> 
> --
> 2.31.0