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

[Jason Wang <jasowang@redhat.com>]

在 2021/4/1 下午5:58, Eric Dumazet 写道:
> 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.


So I wonder something like the following like this help. We know the 
frag size, that means, if we know there's sufficient tailroom we can use 
build_skb() without reserving dedicated room for skb_shared_info.

Thanks


>
> 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
>>>>>>>