netdev - Re: [PATCH vhost v13 05/12] virtio_ring: introduce virtqueue_dma

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Date: Wed, 16 Aug 2023 10:19:34 +0800
From: Jason Wang <jasowang@...hat.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: virtualization@...ts.linux-foundation.org, 
	"Michael S. Tsirkin" <mst@...hat.com>, "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>, 
	John Fastabend <john.fastabend@...il.com>, netdev@...r.kernel.org, bpf@...r.kernel.org, 
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH vhost v13 05/12] virtio_ring: introduce virtqueue_dma_dev()

On Wed, Aug 16, 2023 at 10:16 AM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
>
> On Wed, 16 Aug 2023 09:13:48 +0800, Jason Wang <jasowang@...hat.com> wrote:
> > On Tue, Aug 15, 2023 at 5:40 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> > >
> > > On Tue, 15 Aug 2023 15:50:23 +0800, Jason Wang <jasowang@...hat.com> wrote:
> > > > On Tue, Aug 15, 2023 at 2:32 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> > > > >
> > > > >
> > > > > Hi, Jason
> > > > >
> > > > > Could you skip this patch?
> > > >
> > > > I'm fine with either merging or dropping this.
> > > >
> > > > >
> > > > > Let we review other patches firstly?
> > > >
> > > > I will be on vacation soon, and won't have time to do this until next week.
> > >
> > > Have a happly vacation.
> > >
> > > >
> > > > But I spot two possible "issues":
> > > >
> > > > 1) the DMA metadata were stored in the headroom of the page, this
> > > > breaks frags coalescing, we need to benchmark it's impact
> > >
> > > Not every page, just the first page of the COMP pages.
> > >
> > > So I think there is no impact.
> >
> > Nope, see this:
> >
> >         if (SKB_FRAG_PAGE_ORDER &&
> >             !static_branch_unlikely(&net_high_order_alloc_disable_key)) {
> >                 /* Avoid direct reclaim but allow kswapd to wake */
> >                 pfrag->page = alloc_pages((gfp & ~__GFP_DIRECT_RECLAIM) |
> >                                           __GFP_COMP | __GFP_NOWARN |
> >                                           __GFP_NORETRY,
> >                                           SKB_FRAG_PAGE_ORDER);
> >                 if (likely(pfrag->page)) {
> >                         pfrag->size = PAGE_SIZE << SKB_FRAG_PAGE_ORDER;
> >                         return true;
> >                 }
> >         }
> >
> > The comp page might be disabled due to the SKB_FRAG_PAGE_ORDER and
> > net_high_order_alloc_disable_key.
>
>
> YES.
>
> But if comp page is disabled. Then we only get one page each time. The pages are
> not contiguous, so we don't have frags coalescing.
>
> If you mean the two pages got from alloc_page may be contiguous. The coalescing
> may then be broken. It's a possibility, but I think the impact will be small.

Let's have a simple benchmark and see?

Thanks

>
> Thanks.
>
>
> >
> > >
> > >
> > > > 2) pre mapped DMA addresses were not reused in the case of XDP_TX/XDP_REDIRECT
> > >
> > > Because that the tx is not the premapped mode.
> >
> > Yes, we can optimize this on top.
> >
> > Thanks
> >
> > >
> > > Thanks.
> > >
> > > >
> > > > I see Michael has merge this series so I'm fine to let it go first.
> > > >
> > > > Thanks
> > > >
> > > > >
> > > > > Thanks.
> > > > >
> > > >
> > >
> >
>