linux-kernel - Re: [PATCH] perf: replace automatic const char[] variables by statics

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Message-ID: <20190111133202.GS22483@kernel.org>
Date:   Fri, 11 Jan 2019 10:32:02 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf: replace automatic const char[] variables by statics

Em Sat, Nov 03, 2018 at 12:06:23AM +0100, Rasmus Villemoes escreveu:
> An automatic const char[] variable gets initialized at runtime, just
> like any other automatic variable. For long strings, that uses a lot of
> stack and wastes time building the string; e.g. for the "No %s
> allocation events..." case one has
> 
>   444516:       48 b8 4e 6f 20 25 73 20 61 6c   movabs $0x6c61207325206f4e,%rax # "No %s al"
>   ...
>   444674:       48 89 45 80                     mov    %rax,-0x80(%rbp)
>   444678:       48 b8 6c 6f 63 61 74 69 6f 6e   movabs $0x6e6f697461636f6c,%rax # "location"
>   444682:       48 89 45 88                     mov    %rax,-0x78(%rbp)
>   444686:       48 b8 20 65 76 65 6e 74 73 20   movabs $0x2073746e65766520,%rax # " events "
>   444690:       66 44 89 55 c4                  mov    %r10w,-0x3c(%rbp)
>   444695:       48 89 45 90                     mov    %rax,-0x70(%rbp)
>   444699:       48 b8 66 6f 75 6e 64 2e 20 20   movabs $0x20202e646e756f66,%rax
> 
> Make them all static so that the compiler just references objects in .rodata.
> 

Ok, using dwarves's codiff tool:

  $ codiff --functions /tmp/perf.before ~/bin/perf
builtin-sched.c:
  cmd_sched                 |  -48
 1 function changed, 48 bytes removed, diff: -48

builtin-report.c:
  cmd_report                |  -32
 1 function changed, 32 bytes removed, diff: -32

builtin-kmem.c:
  cmd_kmem                  |  -64
  build_alloc_func_list     |  -50
 2 functions changed, 114 bytes removed, diff: -114

builtin-c2c.c:
  perf_c2c__report          | -390
 1 function changed, 390 bytes removed, diff: -390

ui/browsers/header.c:
  tui__header_window        | -104
 1 function changed, 104 bytes removed, diff: -104

/home/acme/bin/perf:
 9 functions changed, 688 bytes removed, diff: -688

Thanks, applied.

- Arnaldo