netdev - Re: [PATCH net-next v2 0/4] net: mitigate retpoline overhead

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Date:   Thu, 06 Dec 2018 22:24:09 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     pabeni@...hat.com
Cc:     netdev@...r.kernel.org, eric.dumazet@...il.com, pjt@...gle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v2 0/4] net: mitigate retpoline overhead

From: Paolo Abeni <pabeni@...hat.com>
Date: Wed,  5 Dec 2018 19:13:38 +0100

> The spectre v2 counter-measures, aka retpolines, are a source of measurable
> overhead[1]. We can partially address that when the function pointer refers to
> a builtin symbol resorting to a list of tests vs well-known builtin function and
> direct calls.
> 
> Experimental results show that replacing a single indirect call via
> retpoline with several branches and a direct call gives performance gains
> even when multiple branches are added - 5 or more, as reported in [2].
> 
> This may lead to some uglification around the indirect calls. In netconf 2018
> Eric Dumazet described a technique to hide the most relevant part of the needed
> boilerplate with some macro help.
> 
> This series is a [re-]implementation of such idea, exposing the introduced
> helpers in a new header file. They are later leveraged to avoid the indirect
> call overhead in the GRO path, when possible.
> 
> Overall this gives > 10% performance improvement for UDP GRO benchmark and
> smaller but measurable for TCP syn flood.
> 
> The added infra can be used in follow-up patches to cope with retpoline overhead
> in other points of the networking stack (e.g. at the qdisc layer) and possibly
> even in other subsystems.
 ...

Series applied, thanks!