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

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Message-ID: <1d8801d4-73a9-4822-adf9-20e6c5a6a25c@redhat.com>
Date: Mon, 10 Feb 2025 17:16:47 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Eric Dumazet <edumazet@...gle.com>,
 Willem de Bruijn <willemdebruijn.kernel@...il.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 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?

Side note: FWIW I think we will have 2 holes, 4 bytes each, one after
`sk_forward_alloc` and another one after `sk_mark`.

I missed that explicit alignment of the `tcp_sock_write_tx` group; that
will prevent the overall grow of `struct tcp_sock`, and will avoid bad
side effects while changing the struct layout.

I expect the change you propose would perform alike the RFC patches, but
I'll try to do an explicit test later (and report here the results).

Thanks,

Paolo