netdev - Re: [RFC PATCH 0/2] udp: avoid false sharing on sk

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Message-ID: <1cc7d4f5-8004-43d4-b401-fdaebcf87b45@redhat.com>
Date: Mon, 10 Feb 2025 21:54:40 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 Eric Dumazet <edumazet@...gle.com>
Cc: netdev@...r.kernel.org, Kuniyuki Iwashima <kuniyu@...zon.com>,
 "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
 Simon Horman <horms@...nel.org>, Neal Cardwell <ncardwell@...gle.com>,
 David Ahern <dsahern@...nel.org>
Subject: Re: [RFC PATCH 0/2] udp: avoid false sharing on sk_tsflags

On 2/10/25 6:53 PM, Willem de Bruijn wrote:
> Eric Dumazet wrote:
>> On Mon, Feb 10, 2025 at 5:16 PM Paolo Abeni <pabeni@...hat.com> wrote:
>>>
>>> On 2/10/25 4:13 PM, Eric Dumazet wrote:
>>>> On Mon, Feb 10, 2025 at 5:00 AM Willem de Bruijn
>>>> <willemdebruijn.kernel@...il.com> wrote:
>>>>>
>>>>> Paolo Abeni wrote:
>>>>>> While benchmarking the recently shared page frag revert, I observed a
>>>>>> lot of cache misses in the UDP RX path due to false sharing between the
>>>>>> sk_tsflags and the sk_forward_alloc sk fields.
>>>>>>
>>>>>> Here comes a solution attempt for such a problem, inspired by commit
>>>>>> f796feabb9f5 ("udp: add local "peek offset enabled" flag").
>>>>>>
>>>>>> The first patch adds a new proto op allowing protocol specific operation
>>>>>> on tsflags updates, and the 2nd one leverages such operation to cache
>>>>>> the problematic field in a cache friendly manner.
>>>>>>
>>>>>> The need for a new operation is possibly suboptimal, hence the RFC tag,
>>>>>> but I could not find other good solutions. I considered:
>>>>>> - moving the sk_tsflags just before 'sk_policy', in the 'sock_read_rxtx'
>>>>>>   group. It arguably belongs to such group, but the change would create
>>>>>>   a couple of holes, increasing the 'struct sock' size and would have
>>>>>>   side effects on other protocols
>>>>>> - moving the sk_tsflags just before 'sk_stamp'; similar to the above,
>>>>>>   would possibly reduce the side effects, as most of 'struct sock'
>>>>>>   layout will be unchanged. Could increase the number of cacheline
>>>>>>   accessed in the TX path.
>>>>>>
>>>>>> I opted for the present solution as it should minimize the side effects
>>>>>> to other protocols.
>>>>>
>>>>> The code looks solid at a high level to me.
>>>>>
>>>>> But if the issue can be adddressed by just moving a field, that is
>>>>> quite appealing. So have no reviewed closely yet.
>>>>>
>>>>
>>>> sk_tsflags has not been put in an optimal group, I would indeed move it,
>>>> even if this creates one hole.
>>>>
>>>> Holes tend to be used quite fast anyway with new fields.
>>>>
>>>> Perhaps sock_read_tx group would be the best location,
>>>> because tcp_recv_timestamp() is not called in the fast path.
>>>
>>> Just to wrap my head on the above reasoning: for UDP such a change could
>>> possibly increase the number of `struct sock` cache-line accessed in the
>>> RX path (the `sock_write_tx` group should not be touched otherwise) but
>>> that will not matter much, because we expect a low number of UDP sockets
>>> in the system, right?
>>
>> Are you referring to UDP applications needing timestamps ?
>>
>> Because sk_tsflags is mostly always used in TX
> 
> I thought the issue on rx was with the test in sock_recv_cmsgs.

Yes, the critical bits in my tests are in sock_recv_cmsgs(): such
function touches sk->sk_tsflags unconditionally on each recvmsg call.

/P