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

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On Mon, 10 Jan 2022 08:50:08 -0500, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> 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 <mst@xxxxxxxxxx> 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.
>

I see what you mean, I will try to reduce the use of indirect.
And bring performance test data.

Thanks for your opinion.

>
>
> > >
> > > > 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
> > >
>
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