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Date:   Mon, 11 Jan 2021 14:04:18 -0700
From:   Nathan Chancellor <natechancellor@gmail.com>
To:     Bill Wendling <morbo@google.com>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Masahiro Yamada <masahiroy@kernel.org>,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-kbuild@vger.kernel.org, clang-built-linux@googlegroups.com,
        Andrew Morton <akpm@linux-foundation.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Sami Tolvanen <samitolvanen@google.com>
Subject: Re: [PATCH] pgo: add clang's Profile Guided Optimization
 infrastructure
Message-ID: <20210111210418.GA3660906@ubuntu-m3-large-x86>
References: <20210111081821.3041587-1-morbo@google.com>
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On Mon, Jan 11, 2021 at 12:18:21AM -0800, Bill Wendling wrote:
> From: Sami Tolvanen <samitolvanen@google.com>
> 
> Enable the use of clang's Profile-Guided Optimization[1]. To generate a
> profile, the kernel is instrumented with PGO counters, a representative
> workload is run, and the raw profile data is collected from
> /sys/kernel/debug/pgo/profraw.
> 
> The raw profile data must be processed by clang's "llvm-profdata" tool before
> it can be used during recompilation:
> 
>   $ cp /sys/kernel/debug/pgo/profraw vmlinux.profraw
>   $ llvm-profdata merge --output=vmlinux.profdata vmlinux.profraw
> 
> Multiple raw profiles may be merged during this step.
> 
> The data can be used either by the compiler if LTO isn't enabled:
> 
>     ... -fprofile-use=vmlinux.profdata ...
> 
> or by LLD if LTO is enabled:
> 
>     ... -lto-cs-profile-file=vmlinux.profdata ...
> 
> This initial submission is restricted to x86, as that's the platform we know
> works. This restriction can be lifted once other platforms have been verified
> to work with PGO.
> 
> Note that this method of profiling the kernel is clang-native and isn't
> compatible with clang's gcov support in kernel/gcov.
> 
> [1] https://clang.llvm.org/docs/UsersManual.html#profile-guided-optimization
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> Co-developed-by: Bill Wendling <morbo@google.com>
> Signed-off-by: Bill Wendling <morbo@google.com>

I took this for a spin against x86_64_defconfig and ran into two issues:

1. https://github.com/ClangBuiltLinux/linux/issues/1252

   There is also one in drivers/gpu/drm/i915/i915_query.c. For the time
   being, I added PGO_PROFILE_... := n for those two files.

2. After doing that, I run into an undefined function error with ld.lld.

How I tested:

$ make -skj"$(nproc)" LLVM=1 defconfig

$ scripts/config -e PGO_CLANG

$ make -skj"$(nproc)" LLVM=1 olddefconfig vmlinux all
ld.lld: error: undefined symbol: __llvm_profile_instrument_memop
>>> referenced by head64.c
>>>               arch/x86/kernel/head64.o:(__early_make_pgtable)
>>> referenced by head64.c
>>>               arch/x86/kernel/head64.o:(x86_64_start_kernel)
>>> referenced by head64.c
>>>               arch/x86/kernel/head64.o:(copy_bootdata)
>>> referenced 2259 more times

Local diff:

diff --git a/drivers/char/Makefile b/drivers/char/Makefile
index ffce287ef415..4b2f238770b5 100644
--- a/drivers/char/Makefile
+++ b/drivers/char/Makefile
@@ -4,6 +4,7 @@
 #
 
 obj-y				+= mem.o random.o
+PGO_PROFILE_random.o		:= n
 obj-$(CONFIG_TTY_PRINTK)	+= ttyprintk.o
 obj-y				+= misc.o
 obj-$(CONFIG_ATARI_DSP56K)	+= dsp56k.o
diff --git a/drivers/gpu/drm/i915/Makefile b/drivers/gpu/drm/i915/Makefile
index e5574e506a5c..d83cacc79b1a 100644
--- a/drivers/gpu/drm/i915/Makefile
+++ b/drivers/gpu/drm/i915/Makefile
@@ -168,6 +168,7 @@ i915-y += \
 	  i915_vma.o \
 	  intel_region_lmem.o \
 	  intel_wopcm.o
+PGO_PROFILE_i915_query.o := n
 
 # general-purpose microcontroller (GuC) support
 i915-y += gt/uc/intel_uc.o \