linux-kernel - Re: [PATCH v2 37/59] x86/putuser: Provide room for padding

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Message-ID: <CAHk-=wj-njE+DwycFcVQ=ouXZpgKU2rjG3G0keNXSFWa_etrEA@mail.gmail.com>
Date:   Fri, 2 Sep 2022 14:46:08 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Tim Chen <tim.c.chen@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Johannes Wikner <kwikner@...z.ch>,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Jann Horn <jannh@...gle.com>, "H.J. Lu" <hjl.tools@...il.com>,
        Joao Moreira <joao.moreira@...el.com>,
        Joseph Nuzman <joseph.nuzman@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Juergen Gross <jgross@...e.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        K Prateek Nayak <kprateek.nayak@....com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH v2 37/59] x86/putuser: Provide room for padding

On Fri, Sep 2, 2022 at 1:25 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> The below (mashup of a handful of patches) is the best I could come up
> with in a hurry.

Hmm. It doesn't look too horrible, but yeah, fi it still ends up
getting the same padding overhead I'm not sure it ends up really
mattering.

I was hoping that some of the overhead would just go away - looking at
my kernel build, we do have a fair number of functions that are 1-31
bytes (according to a random and probably broken little shell script:

    objdump -t vmlinux |
        grep 'F .text' |
        sort |
        cut -f2- |
        grep '^00000000000000[01]'

but from a quick look, a fair number of them aren't actually even leaf
functions (ie they are small because they just call another function
with a set of simple arguments, often as a tail-call).,

So maybe it's just not worth it.

> If this patch makes you feel warm and fuzzy, I can clean it up,
> otherwise I would suggest getting the stuff we have merged before adding
> even more things on top.

Yeah, it doesn't look that convincing.  I have no real numbers - a lot
of small functions, but I'm not convinced that it is worth worrying
about them, particularly if it doesn't really help the actual text
size.

              Linus