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Date:   Sat, 13 Mar 2021 12:23:28 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Sedat Dilek <sedat.dilek@gmail.com>
Cc:     Peter Zijlstra <peterz@infradead.org>, x86@kernel.org,
        rostedt@goodmis.org, hpa@zytor.com, torvalds@linuxfoundation.org,
        linux-kernel@vger.kernel.org, linux-toolchains@vger.kernel.org,
        jpoimboe@redhat.com, alexei.starovoitov@gmail.com,
        mhiramat@kernel.org
Subject: Re: [PATCH 0/2] x86: Remove ideal_nops[]
Message-ID: <20210313112328.GB16144@zn.tnic>
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On Sat, Mar 13, 2021 at 09:49:23AM +0100, Borislav Petkov wrote:
> Lemme rerun here with clang.

clang11 is almost twice as slow as gcc but difference is still
negligible: ~0.6 seconds.

./tools/perf/perf stat --repeat 5 --sync --pre=/root/bin/pre-build-kernel.sh -- make -s -j9 LLVM=1 LLVM_IAS=1 bzImage

before:

 Performance counter stats for 'make -s -j9 LLVM=1 LLVM_IAS=1 bzImage' (5 runs):

      5,576,081.48 msec task-clock                #    7.664 CPUs utilized            ( +-  0.03% )
           496,841      context-switches          #    0.089 K/sec                    ( +-  0.11% )
            30,245      cpu-migrations            #    0.005 K/sec                    ( +-  0.53% )
        49,702,714      page-faults               #    0.009 M/sec                    ( +-  0.00% )
19,954,704,926,347      cycles                    #    3.579 GHz                      ( +-  0.02% )  (83.33%)
15,920,125,996,460      stalled-cycles-frontend   #   79.78% frontend cycles idle     ( +-  0.03% )  (83.33%)
13,177,812,137,935      stalled-cycles-backend    #   66.04% backend cycles idle      ( +-  0.04% )  (66.67%)
 8,778,060,061,848      instructions              #    0.44  insn per cycle
                                                  #    1.81  stalled cycles per insn  ( +-  0.00% )  (83.33%)
 1,852,121,066,032      branches                  #  332.155 M/sec                    ( +-  0.00% )  (83.33%)
    84,048,262,434      branch-misses             #    4.54% of all branches          ( +-  0.02% )  (83.33%)

           727.572 +- 0.305 seconds time elapsed  ( +-  0.04% )

after:

 Performance counter stats for 'make -s -j9 LLVM=1 LLVM_IAS=1 bzImage' (5 runs):

      5,581,654.38 msec task-clock                #    7.665 CPUs utilized            ( +-  0.01% )
           496,274      context-switches          #    0.089 K/sec                    ( +-  0.12% )
            30,645      cpu-migrations            #    0.005 K/sec                    ( +-  0.54% )
        49,711,551      page-faults               #    0.009 M/sec                    ( +-  0.01% )
19,968,933,753,686      cycles                    #    3.578 GHz                      ( +-  0.01% )  (83.33%)
15,925,776,797,854      stalled-cycles-frontend   #   79.75% frontend cycles idle     ( +-  0.01% )  (83.33%)
13,182,158,323,446      stalled-cycles-backend    #   66.01% backend cycles idle      ( +-  0.01% )  (66.67%)
 8,778,619,885,119      instructions              #    0.44  insn per cycle
                                                  #    1.81  stalled cycles per insn  ( +-  0.00% )  (83.33%)
 1,852,096,100,464      branches                  #  331.818 M/sec                    ( +-  0.01% )  (83.33%)
    84,264,257,355      branch-misses             #    4.55% of all branches          ( +-  0.03% )  (83.33%)

          728.2400 +- 0.0613 seconds time elapsed  ( +-  0.01% )

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette