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

[Eric Dumazet <edumazet@google.com>]

On Thu, Apr 1, 2021 at 11:04 AM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>
> On Thu, 1 Apr 2021 15:14:18 +0800, Jason Wang <jasowang@redhat.com> wrote:
> >
> > 在 2021/3/31 下午4:11, Michael S. Tsirkin 写道:
> > > 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?
> >
> >
> > This might work only if the device sets a correct hdr_len. I'm not sure
> > all of the devices can do this properly. E.g for tap, we use
> > skb_headlen() in virtio_net_hdr_from_skb() which depends highly on the
> > behaviour of the underlayer layers (device driver or GRO). And we only
> > set this hint for GSO packet but virtio-net may tries to do GRO for non
> > GSO packets.
> >
> > Thanks
>
> hi, Jason
>
> I personally prefer to use build_skb to create skb, so the problem here is
> actually gone.
>
> The premise of this is that the buffer added by add_recvbuf_mergeable must
> retain a skb_shared_info. Of course, then rx frags coalescing won't
> work. But I consider that suppose the size of the mrg_avg_pkt_len 1500, so we
> can still store 17 * 1500 = 24k packets in a skb. If the packet is really big,
> the mrg_avg_pkt_len will also increase, and the buffer allocated later will
> increase. When the mrg_avg_pkt_len is greater than PAGE_SIZE/2, rx frags
> coalesce is no longer needed. Because we can't allocate two bufs with a value of
> mrg_avg_pkt_len on the same page.
>

For the record I implemented build_skb() 10 years ago, so you can
trust me when I
am saying this will not help.

Using build_skb() will waste additional skb_shared_info per MSS.
That's an increase of 20% of memory, for nothing at all.

Also there are cases when this won't be possible, say if you use an MTU of 4000





> Thanks.
>
> >
> >
> > >
> > >
> > >> 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: su-lifan@linux.alibaba.com
> > >>>
> > >>>> Fixes: fd11a83dd363 ("net: Pull out core bits of __netdev_alloc_skb and add __napi_alloc_skb")
> > >>>> Signed-off-by: Eric Dumazet <edumazet@google.com>
> > >>>> Cc: Alexander Duyck <alexanderduyck@fb.com>
> > >>>> Cc: Paolo Abeni <pabeni@redhat.com>
> > >>>> Cc: Michael S. Tsirkin <mst@redhat.com>
> > >>>> Cc: Greg Thelen <gthelen@google.com>
> > >>>> ---
> > >>>>   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
> > >>>>
> >