linux-kernel - Re: [PATCH v1 0/7] DCE/DSE: Add Dead Syscalls Elimination support, part1

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Message-Id: <a08e1cee-05e0-447a-b25b-6c2ebb116e8d@app.fastmail.com>
Date:   Tue, 26 Sep 2023 13:24:21 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Zhangjin Wu" <falcon@...ylab.org>, linux-kernel@...r.kernel.org,
        linux-mips@...r.kernel.org, linux-riscv@...ts.infradead.org
Cc:     "Palmer Dabbelt" <palmer@...osinc.com>,
        "Paul Walmsley" <paul.walmsley@...ive.com>, paulburton@...nel.org,
        "Paul E. McKenney" <paulmck@...nel.org>,
        "Thomas Bogendoerfer" <tsbogend@...ha.franken.de>,
        "Willy Tarreau" <w@....eu>,
        Thomas Weißschuh <linux@...ssschuh.net>,
        "Tim Bird" <tim.bird@...y.com>,
        "Geert Uytterhoeven" <geert@...ux-m68k.org>,
        "Nicolas Pitre" <nico@...xnic.net>
Subject: Re: [PATCH v1 0/7] DCE/DSE: Add Dead Syscalls Elimination support, part1

On Tue, Sep 26, 2023, at 09:14, Arnd Bergmann wrote:
> On Tue, Sep 26, 2023, at 00:33, Zhangjin Wu wrote:
>
> It would be nice to include some size numbers here for at least
> one practical use case. If you have a defconfig for a shipping
> product with a small kernel, what is the 'size -B' output you
> see comparing with and without DCE and, and with DCE+DSE?

To follow up on this myself, for a very rough baseline,
I tried a riscv tinyconfig build with and without 
CONFIG_LD_DEAD_CODE_DATA_ELIMINATION (this is currently
not supported on arm, so I did not try it there), and
then another build with simply *all* system calls stubbed
out by hacking asm/syscall-wrapper.h:

$ size build/tmp/vmlinux-*
   text	   data	    bss	     dec    hex	filename
  754772  220016  71841	 1046629  ff865	vmlinux-tinyconfig
  717500  223368  71841	 1012709  f73e5	vmlinux-tiny+nosyscalls
  567310  176200  71473	  814983  c6f87	vmlinux-tiny+gc-sections
  493278  170752  71433	  735463  b38e7	vmlinux-tiny+gc-sections+nosyscalls
10120058 3572756 493701	14186515 d87813	vmlinux-defconfig
 9953934 3529004 491525	13974463 d53bbf	vmlinux-defconfig+gc
 9709856 3500600 489221	13699677 d10a5d	vmlinux-defconfig+gc+nosyscalls

This would put us at an upper bound of 10% size savings (80kb) for
tinyconfig, which is clearly significant. For defconfig, it's
still 2.0% or 275kb size reduction when all syscalls are dropped.

     Arnd