Re: [PATCH net] net: avoid 32 x truesize under-estimation for tiny skbs

[Eric Dumazet <edumazet@google.com>]

On Wed, Mar 31, 2021 at 10:36 AM Eric Dumazet <edumazet@google.com> wrote:
>
> On Wed, Mar 31, 2021 at 10:11 AM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Mon, Mar 29, 2021 at 11:06:09AM +0200, Eric Dumazet wrote:
> > > On Mon, Mar 29, 2021 at 10:52 AM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> > > >
> > > > On Wed, 13 Jan 2021 08:18:19 -0800, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > > > > From: Eric Dumazet <edumazet@google.com>
> > > > >
> > > > > Both virtio net and napi_get_frags() allocate skbs
> > > > > with a very small skb->head
> > > > >
> > > > > While using page fragments instead of a kmalloc backed skb->head might give
> > > > > a small performance improvement in some cases, there is a huge risk of
> > > > > under estimating memory usage.
> > > > >
> > > > > For both GOOD_COPY_LEN and GRO_MAX_HEAD, we can fit at least 32 allocations
> > > > > per page (order-3 page in x86), or even 64 on PowerPC
> > > > >
> > > > > We have been tracking OOM issues on GKE hosts hitting tcp_mem limits
> > > > > but consuming far more memory for TCP buffers than instructed in tcp_mem[2]
> > > > >
> > > > > Even if we force napi_alloc_skb() to only use order-0 pages, the issue
> > > > > would still be there on arches with PAGE_SIZE >= 32768
> > > > >
> > > > > This patch makes sure that small skb head are kmalloc backed, so that
> > > > > other objects in the slab page can be reused instead of being held as long
> > > > > as skbs are sitting in socket queues.
> > > > >
> > > > > Note that we might in the future use the sk_buff napi cache,
> > > > > instead of going through a more expensive __alloc_skb()
> > > > >
> > > > > Another idea would be to use separate page sizes depending
> > > > > on the allocated length (to never have more than 4 frags per page)
> > > > >
> > > > > I would like to thank Greg Thelen for his precious help on this matter,
> > > > > analysing crash dumps is always a time consuming task.
> > > >
> > > >
> > > > This patch causes a performance degradation of about 10% in the scenario of
> > > > virtio-net + GRO.
> > > >
> > > > For GRO, there is no way to merge skbs based on frags with this patch, only
> > > > frag_list can be used to link skbs. The problem that this cause are that compared
> > > > to the GRO package merged into the frags way, the current skb needs to call
> > > > kfree_skb_list to release each skb, resulting in performance degradation.
> > > >
> > > > virtio-net will store some data onto the linear space after receiving it. In
> > > > addition to the header, there are also some payloads, so "headlen <= offset"
> > > > fails. And skb->head_frag is failing when use kmalloc() for skb->head allocation.
> > > >
> > >
> > > Thanks for the report.
> > >
> > > There is no way we can make things both fast for existing strategies
> > > used by _insert_your_driver
> > > and malicious usages of data that can sit for seconds/minutes in socket queues.
> > >
> > > I think that if you want to gain this 10% back, you have to change
> > > virtio_net to meet optimal behavior.
> > >
> > > Normal drivers make sure to not pull payload in skb->head, only headers.
> >
> > Hmm we do have hdr_len field, but seem to ignore it on RX.
> > Jason do you see any issues with using it for the head len?
> >
>
> I was looking at this code (page_to_skb())  a few minutes ago ;)
>
> pulling payload would make sense only if can pull of of it (to free the page)
> (This is what some drivers implement and call copybreak)
>
> Even if we do not have an accurate knowledge of header sizes,
> it would be better to pull only the Ethernet header and let GRO do the
> rest during its dissection.
>
> Once fixed, virtio_net will reduce by 2x number of frags per skb,
> compared to the situation before "net: avoid 32 x truesize
> under-estimation for tiny skbs"

Ie I suspect the simple way to fix this would be :

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index bb4ea9dbc16bcb19c5969fc8247478aa66c63fce..a5500bf6ac01051be949edf9fead934a90335f4f
100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -409,9 +409,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
        offset += hdr_padded_len;
        p += hdr_padded_len;

-       copy = len;
-       if (copy > skb_tailroom(skb))
-               copy = skb_tailroom(skb);
+       copy = min_t(int, len, ETH_HLEN);
        skb_put_data(skb, p, copy);

        if (metasize) {