Re: [PATCH v2] tcp: Disable Nagle's algorithm (set TCP_NODELAY) on all sockets

On Tue, Jan 21, 2025 at 09:40:02AM +0100, Stefano Brivio wrote:
...
  On Tue, 21 Jan 2025 13:31:03 +1030
 David Gibson <david(a)gibson.dropbear.id.au> wrote:
 
  On Mon, Jan 20, 2025 at 06:28:16PM +0100, Stefano
Brivio wrote:
  Following up on 725acd111ba3 ("tcp_splice:
Set (again) TCP_NODELAY on
 both sides"), David argues that, in general, we don't know what kind
 of TCP traffic we're dealing with, on any side or path.
 
 TCP segments might have been delivered to our socket with a PSH flag,
 but we don't have a way to know about it.
 
 Similarly, the guest might send us segments with PSH or URG set, but
 we don't know if we should generally TCP_CORK sockets and uncork on
 those flags, because that would assume they're running a Linux kernel
 (and a particular version of it) matching the kernel that delivers
 outbound packets for us.
 
 Given that we can't make any assumption and everything might very well
 be interactive traffic, disable Nagle's algorithm on all non-spliced
 sockets as well.
 
 After all, John Nagle himself is nowadays recommending that delayed
 ACKs should never be enabled together with his algorithm, but we
 don't have a practical way to ensure that our environment is free from
 delayed ACKs (TCP_QUICKACK is not really usable for this purpose):
 
   https://news.ycombinator.com/item?id=34180239
 
 Suggested-by: David Gibson <david(a)gibson.dropbear.id.au>
 Signed-off-by: Stefano Brivio <sbrivio(a)redhat.com>
 ---
 v2: Set TCP_NODELAY on inbound socket after accept4(), not on the
     listening sockets, and change failure message to debug()
     instead of trace()   
 
 Uh.. you've moved it to after accept() for incoming connections, but
 it looks like you've accidentally removed the call for outgoing
 connections (tcp_conn_new_sock()). 
 
 Uh... no? It's here:
 
  >  tcp.c | 15 +++++++++++++++
 >  1 file changed, 15 insertions(+)
 > 
 > diff --git a/tcp.c b/tcp.c
 > index a012b81..4d6a6b3 100644
 > --- a/tcp.c
 > +++ b/tcp.c
 > @@ -756,6 +756,19 @@ static void tcp_sock_set_bufsize(const struct ctx *c, int s)
 >  		trace("TCP: failed to set SO_SNDBUF to %i", v);
 >  }
 >  
 > +/**
 > + * tcp_sock_set_nodelay() - Set TCP_NODELAY option (disable Nagle's
algorithm)
 > + * @s:		Socket, can be -1 to avoid check in the caller
 > + */
 > +static void tcp_sock_set_nodelay(int s)
 > +{
 > +	if (s == -1)
 > +		return;
 > +
 > +	if (setsockopt(s, SOL_TCP, TCP_NODELAY, &((int){ 1 }), sizeof(int)))
 > +		debug("TCP: failed to set TCP_NODELAY on socket %i", s);
 > +}
 > +
 >  /**
 >   * tcp_update_csum() - Calculate TCP checksum
 >   * @psum:	Unfolded partial checksum of the IPv4 or IPv6 pseudo-header
 > @@ -1285,6 +1298,7 @@ static int tcp_conn_new_sock(const struct ctx *c, sa_family_t
af) 
                                      ^^^^^^^^^^^^^^^^^
 
  >  		return -errno;
 >  
 >  	tcp_sock_set_bufsize(c, s);
 > +	tcp_sock_set_nodelay(s); 
         ^^^^^^^^^^^^^^^^^^^^^^^^
 
 ...am I missing something? 
Oops, no, I am.  The page break fell at an inconvenient spot which
made my eye skate over the relevant hunk, sorry.

...
  
  >  
 >  	return s;
 >  }
 > @@ -2058,6 +2072,7 @@ void tcp_listen_handler(const struct ctx *c, union epoll_ref
ref,
 >  		goto cancel;
 >  
 >  	tcp_sock_set_bufsize(c, s);
 > +	tcp_sock_set_nodelay(s);
 >  
 >  	/* FIXME: When listening port has a specific bound address, record that
 >  	 * as our address   
  
-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson