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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 1 Apr 2021 09:51:34 -0400

On Wed, Mar 31, 2021 at 10:36:35AM +0200, Eric Dumazet wrote:
> On Wed, Mar 31, 2021 at 10:11 AM Michael S. Tsirkin <[email-masked]> 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 <[email-masked]> wrote:
> > > >
> > > > On Wed, 13 Jan 2021 08:18:19 -0800, Eric Dumazet <[email-masked]> wrote:
> > > > > From: Eric Dumazet <[email-masked]>
> > > > >
> > > > > 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)

right.. I wonder whether it's preferable to always copy as much as fits,
or to use hdr_len if it's there?

> 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.

So IIUC what you are saying is we should do more or less
     if (hdr_len != 0)
                        copy hdr_len
?

> 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"
> 
> 
> >
> > > Optimal GRO packets are when payload is in page fragments.
> > >
> > > (I am speaking not only for raw performance, but ability for systems
> > > to cope with network outages and sudden increase of memory usage in
> > > out of order queues)
> > >
> > > This has been quite clearly stated in my changelog.
> > >
> > > Thanks.
> > >
> > >
> > > > int skb_gro_receive(struct sk_buff *p, struct sk_buff *skb)
> > > > {
> > > >         struct skb_shared_info *pinfo, *skbinfo = skb_shinfo(skb);
> > > >         unsigned int offset = skb_gro_offset(skb);
> > > >         unsigned int headlen = skb_headlen(skb);
> > > >
> > > >     .......
> > > >
> > > >         if (headlen <= offset) {         // virtio-net will fail
> > > >         ........ // merge by frags
> > > >                 goto done;
> > > >         } else if (skb->head_frag) {     // skb->head_frag is fail when use kmalloc() for skb->head allocation
> > > >         ........ // merge by frags
> > > >                 goto done;
> > > >         }
> > > >
> > > > merge:
> > > >     ......
> > > >
> > > >         if (NAPI_GRO_CB(p)->last == p)
> > > >                 skb_shinfo(p)->frag_list = skb;
> > > >         else
> > > >                 NAPI_GRO_CB(p)->last->next = skb;
> > > >
> > > >     ......
> > > >         return 0;
> > > > }
> > > >
> > > >
> > > > test cmd:
> > > >  for i in $(seq 1 4)
> > > >  do
> > > >     redis-benchmark -r 10000000 -n 10000000 -t set -d 1024 -c 8 -P 32 -h  <ip> -p 6379 2>&1 | grep 'per second'  &
> > > >  done
> > > >
> > > > Reported-by: [email-masked]
> > > >
> > > > >
> > > > > Fixes: fd11a83dd363 ("net: Pull out core bits of __netdev_alloc_skb and add __napi_alloc_skb")
> > > > > Signed-off-by: Eric Dumazet <[email-masked]>
> > > > > Cc: Alexander Duyck <[email-masked]>
> > > > > Cc: Paolo Abeni <[email-masked]>
> > > > > Cc: Michael S. Tsirkin <[email-masked]>
> > > > > Cc: Greg Thelen <[email-masked]>
> > > > > ---
> > > > >  net/core/skbuff.c | 9 +++++++--
> > > > >  1 file changed, 7 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > > > > index 7626a33cce590e530f36167bd096026916131897..3a8f55a43e6964344df464a27b9b1faa0eb804f3 100644
> > > > > --- a/net/core/skbuff.c
> > > > > +++ b/net/core/skbuff.c
> > > > > @@ -501,13 +501,17 @@ EXPORT_SYMBOL(__netdev_alloc_skb);
> > > > >  struct sk_buff *__napi_alloc_skb(struct napi_struct *napi, unsigned int len,
> > > > >                                gfp_t gfp_mask)
> > > > >  {
> > > > > -     struct napi_alloc_cache *nc = this_cpu_ptr(&napi_alloc_cache);
> > > > > +     struct napi_alloc_cache *nc;
> > > > >       struct sk_buff *skb;
> > > > >       void *data;
> > > > >
> > > > >       len += NET_SKB_PAD + NET_IP_ALIGN;
> > > > >
> > > > > -     if ((len > SKB_WITH_OVERHEAD(PAGE_SIZE)) ||
> > > > > +     /* If requested length is either too small or too big,
> > > > > +      * we use kmalloc() for skb->head allocation.
> > > > > +      */
> > > > > +     if (len <= SKB_WITH_OVERHEAD(1024) ||
> > > > > +         len > SKB_WITH_OVERHEAD(PAGE_SIZE) ||
> > > > >           (gfp_mask & (__GFP_DIRECT_RECLAIM | GFP_DMA))) {
> > > > >               skb = __alloc_skb(len, gfp_mask, SKB_ALLOC_RX, NUMA_NO_NODE);
> > > > >               if (!skb)
> > > > > @@ -515,6 +519,7 @@ struct sk_buff *__napi_alloc_skb(struct napi_struct *napi, unsigned int len,
> > > > >               goto skb_success;
> > > > >       }
> > > > >
> > > > > +     nc = this_cpu_ptr(&napi_alloc_cache);
> > > > >       len += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> > > > >       len = SKB_DATA_ALIGN(len);
> > > > >
> > > > > --
> > > > > 2.30.0.284.gd98b1dd5eaa7-goog
> > > > >
> >