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Message-ID: <f3427348-41ee-e0bf-79b6-9ff5120e8110@gmail.com>
Date: Fri, 23 Jun 2023 16:03:46 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net, dsahern@...nel.org,
pabeni@...hat.com, kuba@...nel.org
Subject: Re: [PATCH net-next v2] net/tcp: optimise locking for blocking splice
On 6/23/23 15:17, Eric Dumazet wrote:
> On Fri, Jun 23, 2023 at 2:40 PM Pavel Begunkov <asml.silence@...il.com> wrote:
>>
>> Even when tcp_splice_read() reads all it was asked for, for blocking
>> sockets it'll release and immediately regrab the socket lock, loop
>> around and break on the while check.
>>
>> Check tss.len right after we adjust it, and return if we're done.
>> That saves us one release_sock(); lock_sock(); pair per successful
>> blocking splice read.
>>
>> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
>> ---
>>
>> v2: go with Paolo's suggestion
>> aggressively shrink the patch
>>
>> net/ipv4/tcp.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
>> index 71b42eef9dbf..d56edc2c885f 100644
>> --- a/net/ipv4/tcp.c
>> +++ b/net/ipv4/tcp.c
>> @@ -839,7 +839,7 @@ ssize_t tcp_splice_read(struct socket *sock, loff_t *ppos,
>> tss.len -= ret;
>> spliced += ret;
>>
>> - if (!timeo)
>> + if (!tss.len || !timeo)
>> break;
>> release_sock(sk);
>> lock_sock(sk);
>
> SGTM, thanks.
>
> Reviewed-by: Eric Dumazet <edumazet@...gle.com>
>
> I wonder if the "release_sock();sock_lock();" could be replaced by
> sk_flush_backlog() anyway ?
> Or is there any other reason for this dance ?
Now as you mentioned, it definitely sounds like that. And the code
is 15 years old, perhaps nobody was paying attention.
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 71b42eef9dbf527098963bc03deecf55042e2021..d03d38060944d63d2728a7bf90a5c117b7852d8b
> 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -841,8 +841,7 @@ ssize_t tcp_splice_read(struct socket *sock, loff_t *ppos,
>
> if (!timeo)
> break;
> - release_sock(sk);
> - lock_sock(sk);
> + sk_flush_backlog();
>
> if (sk->sk_err || sk->sk_state == TCP_CLOSE ||
> (sk->sk_shutdown & RCV_SHUTDOWN) ||
--
Pavel Begunkov
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