Date:   Thu, 1 Apr 2021 15:14:18 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>
Cc:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        netdev <netdev@...r.kernel.org>,
        Alexander Duyck <alexanderduyck@...com>,
        Paolo Abeni <pabeni@...hat.com>,
        Greg Thelen <gthelen@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, su-lifan@...ux.alibaba.com,
        "dust.li" <dust.li@...ux.alibaba.com>
Subject: Re: [PATCH net] net: avoid 32 x truesize under-estimation for tiny
 skbs


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@...ux.alibaba.com> wrote:
>>> On Wed, 13 Jan 2021 08:18:19 -0800, Eric Dumazet <eric.dumazet@...il.com> wrote:
>>>> From: Eric Dumazet <edumazet@...gle.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


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

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