netdev - Re: de-indirect TCP congestion control

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Date:   Mon, 12 Mar 2018 13:51:22 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     David Miller <davem@...emloft.net>
Cc:     eric.dumazet@...il.com, netdev@...r.kernel.org
Subject: Re: de-indirect TCP congestion control

On Mon, 12 Mar 2018 16:05:16 -0400 (EDT)
David Miller <davem@...emloft.net> wrote:

> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Mon, 12 Mar 2018 13:03:35 -0700
> 
> > 
> > 
> > On 03/12/2018 12:48 PM, Stephen Hemminger wrote:  
> >> On Mon, 12 Mar 2018 15:04:06 -0400 (EDT)
> >> David Miller <davem@...emloft.net> wrote:
> >>   
> >>> From: Stephen Hemminger <stephen@...workplumber.org>
> >>> Date: Mon, 12 Mar 2018 11:45:52 -0700
> >>>  
> >>>> Since indirect calls are expensive, and now even more so, perhaps we
> >>>> should figure out
> >>>> a way to make the default TCP congestion control hooks into direct
> >>>> calls.
> >>>> 99% of the users just use the single CC module compiled into the
> >>>> kernel.  
> >>>
> >>> Who is this magic user with only one CC algorithm enabled in their
> >>> kernel?  I want to know who this dude is?
> >>>
> >>> I don't think it's going to help much since people will have I think
> >>> at least two algorithms compiled into nearly everyone's tree.
> >>>
> >>> Distributions will enable everything.
> >>>
> >>> Google is going to have at least two algorithms enabled.
> >>>
> >>> etc. etc. etc.
> >>>
> >>> Getting rid of indirect calls is a fine goal, but the precondition you
> >>> are mentioning to achieve this doesn't seem practical at all.  
> >> What I meant is that kernels with N congestion controls, almost all
> >> traffic
> >> uses the default So that path can be optimized. The example I gave
> >> would
> >> have all the others doing the same indirect call.  
> > 
> > I do not understand. What is default_tcp_ops anyway ?
> > 
> > How changes to /proc/sys/net/ipv4/tcp_congestion_control will impact
> > this ?  
> 
> I'm also confused what is being suggested exactly and how this can
> work. :-)

Trying to directly call the fastpath TCP congestion operations for sockets
using the default.  The problem is how to do this and it gets messy with Kconfig
and CPP to deal with as raw tools. Maybe weak symbols could be used.