netdev - Re: [PATCH net-next 1/2] net/tcp: optimise locking for blocking splice

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Date: Wed, 24 May 2023 13:51:17 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
 edumazet@...gle.com, davem@...emloft.net, dsahern@...nel.org, kuba@...nel.org
Subject: Re: [PATCH net-next 1/2] net/tcp: optimise locking for blocking
 splice

On 5/23/23 14:52, Paolo Abeni wrote:
> On Fri, 2023-05-19 at 14:33 +0100, Pavel Begunkov 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>
>> ---
>>   net/ipv4/tcp.c | 8 +++++---
>>   1 file changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
>> index 4d6392c16b7a..bf7627f37e69 100644
>> --- a/net/ipv4/tcp.c
>> +++ b/net/ipv4/tcp.c
>> @@ -789,13 +789,15 @@ ssize_t tcp_splice_read(struct socket *sock, loff_t *ppos,
>>   	 */
>>   	if (unlikely(*ppos))
>>   		return -ESPIPE;
>> +	if (unlikely(!tss.len))
>> +		return 0;
>>   
>>   	ret = spliced = 0;
>>   
>>   	lock_sock(sk);
>>   
>>   	timeo = sock_rcvtimeo(sk, sock->file->f_flags & O_NONBLOCK);
>> -	while (tss.len) {
>> +	while (true) {
>>   		ret = __tcp_splice_read(sk, &tss);
>>   		if (ret < 0)
>>   			break;
>> @@ -835,10 +837,10 @@ ssize_t tcp_splice_read(struct socket *sock, loff_t *ppos,
>>   			}
>>   			continue;
>>   		}
>> -		tss.len -= ret;
>>   		spliced += ret;
>> +		tss.len -= ret;
> 
> The patch LGTM. The only minor thing that I note is that the above
> chunk is not needed. Perhaps avoiding unneeded delta could be worthy.

It keeps it closer to the tss.len test, so I'd leave it for that reason,
but on the other hand the compiler should be perfectly able to optimise it
regardless (i.e. sub;cmp;jcc; vs sub;jcc;). I don't have a hard feeling
on that, can change if you want.

-- 
Pavel Begunkov