linux-kernel - Re: [PATCH v4] pgo: add clang's Profile Guided Optimization infrastructure

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Message-ID: <CA+icZUXsK8nfRQGatBoMAsjxhA=VEgZDsPsEg8o7i7yP+ko2Lg@mail.gmail.com>
Date:   Mon, 18 Jan 2021 01:57:32 +0100
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Nick Desaulniers <nick.desaulniers@...il.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        akpm@...ux-foundation.org,
        Clang-Built-Linux ML <clang-built-linux@...glegroups.com>,
        corbet@....net, linux-doc@...r.kernel.org,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        masahiroy@...nel.org, Bill Wendling <morbo@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v4] pgo: add clang's Profile Guided Optimization infrastructure

On Sat, Jan 16, 2021 at 1:13 AM Nick Desaulniers
<nick.desaulniers@...il.com> wrote:
>
> > On Wed, Jan 13, 2021 at 8:07 PM Nick Desaulniers
> > <ndesaulniers@...gle.com> wrote:
> > >
> > > On Wed, Jan 13, 2021 at 12:55 PM Nathan Chancellor
> > > <natechancellor@...il.com> wrote:
> > > >
> > > > However, I see an issue with actually using the data:
> > > >
> > > > $ sudo -s
> > > > # mount -t debugfs none /sys/kernel/debug
> > > > # cp -a /sys/kernel/debug/pgo/profraw vmlinux.profraw
> > > > # chown nathan:nathan vmlinux.profraw
> > > > # exit
> > > > $ tc-build/build/llvm/stage1/bin/llvm-profdata merge --output=vmlinux.profdata vmlinux.profraw
> > > > warning: vmlinux.profraw: Invalid instrumentation profile data (bad magic)
> > > > error: No profiles could be merged.
> > > >
> > > > Am I holding it wrong? :) Note, this is virtualized, I do not have any
> > > > "real" x86 hardware that I can afford to test on right now.
> > >
> > > Same.
> > >
> > > I think the magic calculation in this patch may differ from upstream
> > > llvm: https://github.com/llvm/llvm-project/blob/49142991a685bd427d7e877c29c77371dfb7634c/llvm/include/llvm/ProfileData/SampleProf.h#L96-L101
> >
> > Err...it looks like it was the padding calculation.  With that fixed
> > up, we can query the profile data to get insights on the most heavily
> > called functions.  Here's what my top 20 are (reset, then watch 10
> > minutes worth of cat videos on youtube while running `find /` and
> > sleeping at my desk).  Anything curious stand out to anyone?
>
> Hello world from my personal laptop whose kernel was rebuilt with
> profiling data!  Wow, I can run `find /` and watch cat videos on youtube
> so fast!  Users will love this! /s
>
> Checking the sections sizes of .text.hot. and .text.unlikely. looks
> good!
>

On each rebuild I need to pass to make ...?

   LLVM=1 -fprofile-use=vmlinux.profdata

Did you try together with passing LLVM_IAS=1 to make?

- Sedat -


> >
> > $ llvm-profdata show -topn=20 /tmp/vmlinux.profraw
> > Instrumentation level: IR  entry_first = 0
> > Total functions: 48970
> > Maximum function count: 62070879
> > Maximum internal block count: 83221158
> > Top 20 functions with the largest internal block counts:
> >   drivers/tty/n_tty.c:n_tty_write, max count = 83221158
> >   rcu_read_unlock_strict, max count = 62070879
> >   _cond_resched, max count = 25486882
> >   rcu_all_qs, max count = 25451477
> >   drivers/cpuidle/poll_state.c:poll_idle, max count = 23618576
> >   _raw_spin_unlock_irqrestore, max count = 18874121
> >   drivers/cpuidle/governors/menu.c:menu_select, max count = 18721624
> >   _raw_spin_lock_irqsave, max count = 18509161
> >   memchr, max count = 15525452
> >   _raw_spin_lock, max count = 15484254
> >   __mod_memcg_state, max count = 14604619
> >   __mod_memcg_lruvec_state, max count = 14602783
> >   fs/ext4/hash.c:str2hashbuf_signed, max count = 14098424
> >   __mod_lruvec_state, max count = 12527154
> >   __mod_node_page_state, max count = 12525172
> >   native_sched_clock, max count = 8904692
> >   sched_clock_cpu, max count = 8895832
> >   sched_clock, max count = 8894627
> >   kernel/entry/common.c:exit_to_user_mode_prepare, max count = 8289031
> >   fpregs_assert_state_consistent, max count = 8287198
> >
> > --
> > Thanks,
> > ~Nick Desaulniers
> >
>
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