Subject: Re: [PATCH v3 0/3] virtio support cache indirect desc

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 10 Jan 2022 08:50:08 -0500

On Thu, Jan 06, 2022 at 08:48:59PM +0800, Xuan Zhuo wrote:
> On Thu, 6 Jan 2022 07:28:31 -0500, Michael S. Tsirkin <[email-masked]> wrote:
> > On Fri, Oct 29, 2021 at 02:28:11PM +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.
> >
> >
> > So where is this going? I really like the performance boost. My concern
> > is that if guest spans NUMA nodes and when handler switches from
> > node to another this will keep reusing the cache from
> > the old node. A bunch of ways were suggested to address this, but
> > even just making the cache per numa node would help.
> >
> 
> In fact, this is the problem I encountered in implementing virtio-net to support
> xdp socket. With virtqueue reset[0] has been merged into virtio spec. I
> am completing this series of work. My plan is:
> 
> 1. virtio support advance dma
> 2. linux kernel/qemu support virtqueue reset
> 3. virtio-net support AF_XDP
> 4. virtio support cache indirect desc
> 
> [0]: https://github.com/oasis-tcs/virtio-spec/issues/124
> 
> Thanks.

OK it's up to you how to prioritize your work.
An idea though: isn't there a way to reduce the use of indirect?
Even with all the caching, it is surely not free.
We made it work better in the past with:

commit e7428e95a06fb516fac1308bd0e176e27c0b9287
    ("virtio-net: put virtio-net header inline with data"). 
and
commit 6ebbc1a6383fe78be3c0961d1475043ac6cc2542
    virtio-net: Set needed_headroom for virtio-net when VIRTIO_F_ANY_LAYOUT is true

can't something similar be done for XDP?

Another idea is to skip indirect even with s/g as number of outstanding
entries is small. The difficulty with this approach is that it has
to be tested across a large number of configurations, including
storage to make sure we don't cause regressions, unless we
are very conservative and only make a small % of entries direct.
Will doing that still help? It looks attractive on paper:
if guest starts outpacing host and ring begins to fill
up to more than say 10% then we switch to allocating indirect
entries which slows guest down.

> >
> > > 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.
> > >
> > > v3:
> > >   pre-allocate per buffer indirect descriptors array
> > >
> > > v2:
> > >   use struct list_head to cache the desc
> > >
> > > *** BLURB HERE ***
> > >
> > > 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     |  11 +++
> > >  drivers/virtio/virtio.c      |   6 ++
> > >  drivers/virtio/virtio_ring.c | 131 ++++++++++++++++++++++++++++++-----
> > >  include/linux/virtio.h       |  14 ++++
> > >  4 files changed, 145 insertions(+), 17 deletions(-)
> > >
> > > --
> > > 2.31.0
> >