netdev - Re: [PATCHSET 0/4] Add support for no-lock sockets

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Date:   Tue, 12 Apr 2022 19:26:21 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Eric Dumazet <eric.dumazet@...il.com>, io-uring@...r.kernel.org,
        netdev@...r.kernel.org, edumazet@...gle.com
Subject: Re: [PATCHSET 0/4] Add support for no-lock sockets

On 4/12/22 6:40 PM, Eric Dumazet wrote:
> 
> On 4/12/22 13:26, Jens Axboe wrote:
>> Hi,
>>
>> If we accept a connection directly, eg without installing a file
>> descriptor for it, or if we use IORING_OP_SOCKET in direct mode, then
>> we have a socket for recv/send that we can fully serialize access to.
>>
>> With that in mind, we can feasibly skip locking on the socket for TCP
>> in that case. Some of the testing I've done has shown as much as 15%
>> of overhead in the lock_sock/release_sock part, with this change then
>> we see none.
>>
>> Comments welcome!
>>
> How BH handlers (including TCP timers) and io_uring are going to run
> safely ? Even if a tcp socket had one user, (private fd opened by a
> non multi-threaded program), we would still to use the spinlock.

But we don't even hold the spinlock over lock_sock() and release_sock(),
just the mutex. And we do check for running eg the backlog on release,
which I believe is done safely and similarly in other places too.

> Maybe I am missing something, but so far your patches make no sense to
> me.

It's probably more likely I'm missing something, since I don't know this
area nearly as well as you. But it'd be great if you could be specific.


-- 
Jens Axboe