Re: [PATCH vhost v13 05/12] virtio_ring: introduce virtqueue_dma_dev()

[Xuan Zhuo <xuanzhuo@linux.alibaba.com>]

On Wed, 16 Aug 2023 10:33:34 +0800, Jason Wang <jasowang@redhat.com> wrote:
> On Wed, Aug 16, 2023 at 10:24 AM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> >
> > On Wed, 16 Aug 2023 10:19:34 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > > On Wed, Aug 16, 2023 at 10:16 AM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> > > >
> > > > On Wed, 16 Aug 2023 09:13:48 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > > > > On Tue, Aug 15, 2023 at 5:40 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> > > > > >
> > > > > > On Tue, 15 Aug 2023 15:50:23 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > > > > > > On Tue, Aug 15, 2023 at 2:32 PM Xuan Zhuo <xuanzhuo@linux.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?
> >
> >
> > That is ok.
> >
> > I think you want to know the perf num with big traffic and the comp page
> > disabled.
>
> Yes.


Hi,

Host:
	for ((i=0; i < 10; ++i)) do sockperf tp -i 192.168.122.100 -t 1000  -m 64000& done
Guest:
	03:23:12 AM     IFACE   rxpck/s   txpck/s    rxkB/s    txkB/s   rxcmp/s   txcmp/s  rxmcst/s   %ifutil
	03:23:13 AM        lo      0.00      0.00      0.00      0.00      0.00      0.00      0.00      0.00
	03:23:13 AM      ens4  61848.00      1.00 3868036.73      0.58      0.00      0.00      0.00      0.00

	tcpdump:
		03:25:01.741563 IP 192.168.122.1.29693 > 192.168.122.100.11111: UDP, length 64000
		03:25:01.741580 IP 192.168.122.1.22239 > 192.168.122.100.11111: UDP, length 64000
		03:25:01.741623 IP 192.168.122.1.22396 > 192.168.122.100.11111: UDP, length 64000

The Guest CPU util is low, every packet is 64000. But the Host vhost process is
100%. So we can not judge by the traffic or the cpu of the Guest.

So I use the kernel without my patches 0635819decaf9d60e6cacfecfebfabe3cbdddafb.

I want to count the frags coalescing num when the comp page is disabled.

	$ sh -x test.sh
	+ sysctl -w net.core.high_order_alloc_disable=1
	net.core.high_order_alloc_disable = 1
	+ sysctl net.core.high_order_alloc_disable
	net.core.high_order_alloc_disable = 1
	+ sleep 5
	+ timeout 5 bpftrace -e 'kprobe: skb_coalesce_rx_frag{@[nsecs/1000/1000/1000]=count()}'
	Attaching 1 probe...



	+ sysctl -w net.core.high_order_alloc_disable=0
	net.core.high_order_alloc_disable = 0
	+ sysctl net.core.high_order_alloc_disable
	net.core.high_order_alloc_disable = 0
	+ sleep 5
	+ timeout 5 bpftrace -e 'kprobe: skb_coalesce_rx_frag{@[nsecs/1000/1000/1000]=count()}'
	Attaching 1 probe...


	@[356]: 167020
	@[361]: 673653
	@[359]: 900844
	@[360]: 912657
	@[358]: 915853
	@[357]: 932245


We can see that the skb_coalesce_rx_frag is not called when comp page is disabled.
If the comp page is enable, there will be many frags coalescing.

So I think that my change will not have impact.

Thanks.




>
> Thanks
>
> >
> > Thanks.
> >
> >
> > >
> > > 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.
> > > > > > > >
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> >
>