linux-kernel - Re: [PATCH] [v2] ethtool: reduce stack usage with clang

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Message-ID: <CAK8P3a3_aqxrpwwZLjoqsaXaE9RaeaF+VA1uEEHzVYSW1HoQ-A@mail.gmail.com>
Date:   Thu, 7 Mar 2019 23:03:44 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     David Miller <davem@...emloft.net>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Jiri Pirko <jiri@...lanox.com>, Wenwen Wang <wang6495@....edu>,
        Michal Kubecek <mkubecek@...e.cz>,
        Networking <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [v2] ethtool: reduce stack usage with clang

On Thu, Mar 7, 2019 at 6:46 PM David Miller <davem@...emloft.net> wrote:
>
> From: Arnd Bergmann <arnd@...db.de>
> Date: Thu,  7 Mar 2019 16:58:35 +0100
>
> > clang inlines the dev_ethtool() more aggressively than gcc does, leading
> > to a larger amount of used stack space:
> >
> > net/core/ethtool.c:2536:24: error: stack frame size of 1216 bytes in function 'dev_ethtool' [-Werror,-Wframe-larger-than=]
> >
> > Marking the sub-functions that require the most stack space as
> > noinline_for_stack gives us reasonable behavior on all compilers.
> >
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> > ---
> > v2: don't annotate dev_ethtool itself, as pointed out by Michal Kubecek
>
> I'll apply this, but as Michal said this is just papering over the problem,
> the aggregate stack allocation is still the same and very large.

Thanks,

I looked at it again and found that ethtool_get_per_queue_coalesce()
and ethtool_set_per_queue_coalesce() in fact use the same stack slots
(as one would hope) when they are both inlined, so you are both right
that this doesn't change as much for the ethtool_set_per_queue()
function as I had hoped.

On the other hand, at least it helps reduce the stack usage for all
the other sub-functions of dev_ethtool(), which now don't pile
on top of the 1216 bytes for the combined function but only
on top of the 272 bytes for the rest of dev_ethtool.

     Arnd