netdev - Re: [PATCH net-next] net: rfs: add a jump label

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Message-ID: <1481221873.4930.123.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Thu, 08 Dec 2016 10:31:13 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, pabeni@...hat.com
Subject: Re: [PATCH net-next] net: rfs: add a jump label

On Thu, 2016-12-08 at 13:19 -0500, David Miller wrote:
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Wed, 07 Dec 2016 08:29:10 -0800
> 
> > From: Eric Dumazet <edumazet@...gle.com>
> > 
> > RFS is not commonly used, so add a jump label to avoid some conditionals
> > in fast path.
> > 
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> 
> Applied, but I wonder how effective this will really be in the long run.

I guess this applies to about all jump labels.

As soon as the attribute is per namespace, we no longer can use them.

A conditional cost really depends on the expression complexity
(including cache line misses)

TCP stack might benefit from jump labels, like sysctl_tcp_low_latency
which is often set to 1 on hosts mostly using epoll()/poll()/select()
instead of blocking read()/recvmsg()