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

[Eric Dumazet <edumazet@google.com>]

On Wed, Mar 31, 2021 at 1:34 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>
> On Wed, 31 Mar 2021 04:11:12 -0400, 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?
> >
>
> Why not add a struct skb_shared_info space when adding merge/big buffer?
> So that we can use build_skb to create skb. For skb with little data, we can
> copy directly to save pages.
>

This does not matter. build_skb() is a distraction here.

Ultimately, you want to receive fat GRO packets with 17 MSS per sk_buff

The patch I gave earlier will combine :

1) Advantage of having skb->head being backed by slab allocations,
   to solve the OOM issue my patch already coped with.

2)  Up to 17 page frags attached to the sk_buff, storing 17 MSS.

In the past, virtio_net had to use 2 slots in shared_info per MSS
  -One slot for the first ~200 bytes of payload
 - One slot for the last part of the MSS.

Please try my patch, so that we confirm the issue is really in page_to_skb()

Thanks