Messages in this thread | | | From | Marco Elver <> | Date | Wed, 27 May 2020 15:30:08 +0200 | Subject | Re: [PATCH -tip] compiler_types.h: Optimize __unqual_scalar_typeof compilation time |
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On Wed, 27 May 2020 at 15:11, Sedat Dilek <sedat.dilek@gmail.com> wrote: > > On Wed, May 27, 2020 at 2:50 PM Arnd Bergmann <arnd@arndb.de> wrote: > > > > On Wed, May 27, 2020 at 2:35 PM Sedat Dilek <sedat.dilek@gmail.com> wrote: > > > On Wed, May 27, 2020 at 2:31 PM Arnd Bergmann <arnd@arndb.de> wrote: > > > > On Wed, May 27, 2020 at 1:36 PM Sedat Dilek <sedat.dilek@gmail.com> wrote: > > > > > On Wed, May 27, 2020 at 1:27 PM Arnd Bergmann <arnd@arndb.de> wrote: > > > > > > On Wed, May 27, 2020 at 12:33 PM Marco Elver <elver@google.com> wrote: > > > > > > > > > > > > This gives us back 80% of the performance drop on clang, and 50% > > > > > > of the drop I saw with gcc, compared to current mainline. > > > > > > > > > > > > Tested-by: Arnd Bergmann <arnd@arndb.de> > > > > > > > > > > > > > > > > Hi Arnd, > > > > > > > > > > with "mainline" you mean Linux-next aka Linux v5.8 - not v5.7? > > > > > > > > I meant v5.7. > > > > > > > > > I have not seen __unqual_scalar_typeof(x) in compiler_types.h in Linux v5.7. > > > > > > > > > > Is there a speedup benefit also for Linux v5.7? > > > > > Which patches do I need? > > > > > > > > v5.7-rc is the baseline and is the fastest I currently see. On certain files, > > > > I saw an intermittent 10x slowdown that was already fixed earlier, now > > > > linux-next > > > > is more like 2x slowdown for me and 1.2x with this patch on top, so we're > > > > almost back to the speed of linux-5.7. > > > > > > > > > > Which clang version did you use - and have you set KCSAN kconfigs - > > > AFAICS this needs clang-11? > > > > I'm currently using clang-11, but I see the same problem with older > > versions, and both with and without KCSAN enabled. I think the issue > > is mostly the deep nesting of macros that leads to code bloat. > > > > Thanks. > > With clang-10: > > $ scripts/diffconfig /boot/config-5.7.0-rc7-2-amd64-clang .config > BUILD_SALT "5.7.0-rc7-2-amd64-clang" -> "5.7.0-rc7-3-amd64-clang" > +HAVE_ARCH_KCSAN y
Clang 10 doesn't support KCSAN (HAVE_KCSAN_COMPILER unset).
> With clang-11: > > $ scripts/diffconfig /boot/config-5.7.0-rc7-2-amd64-clang .config > BUILD_SALT "5.7.0-rc7-2-amd64-clang" -> "5.7.0-rc7-3-amd64-clang" > CLANG_VERSION 100001 -> 110000 > +CC_HAS_ASM_INLINE y > +HAVE_ARCH_KCSAN y > +HAVE_KCSAN_COMPILER y > +KCSAN n > > Which KCSAN kconfigs did you enable?
To clarify: as said in [1], KCSAN (or any other instrumentation) is no longer relevant to the issue here, and the compile-time regression is observable with most configs. The problem is due to pre-processing and parsing, which came about due to new READ_ONCE() and the __unqual_scalar_typeof() macro (which this patch optimizes).
KCSAN and new ONCEs got tangled up because we first attempted to annotate {READ,WRITE}_ONCE() with data_race(), but that turned out to have all kinds of other issues (explanation in [2]). So we decided to drop all the KCSAN-specific bits from ONCE, and require KCSAN to be Clang 11. Those fixes were applied to the first version of new {READ,WRITE}_ONCE() in -tip, which actually restored the new ONCEs to the pre-KCSAN version (now that KCSAN can deal with them without annotations).
Hope this makes more sense now.
[1] https://lore.kernel.org/lkml/CANpmjNOUdr2UG3F45=JaDa0zLwJ5ukPc1MMKujQtmYSmQnjcXg@mail.gmail.com/ [2] https://lore.kernel.org/lkml/20200521142047.169334-1-elver@google.com/
Thanks, -- Marco
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