Messages in this thread Patch in this message | | | Date | Tue, 26 May 2020 19:33:12 +0200 | From | Marco Elver <> | Subject | Re: [PATCH -tip v3 09/11] data_race: Avoid nested statement expression |
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On Tue, 26 May 2020, Marco Elver wrote:
> On Tue, 26 May 2020 at 14:19, Arnd Bergmann <arnd@arndb.de> wrote: > > > > On Tue, May 26, 2020 at 2:02 PM Will Deacon <will@kernel.org> wrote: > > > On Tue, May 26, 2020 at 12:42:16PM +0200, Arnd Bergmann wrote: > > > > > > > > I find this patch only solves half the problem: it's much faster than > > > > without the > > > > patch, but still much slower than the current mainline version. As far as I'm > > > > concerned, I think the build speed regression compared to mainline is not yet > > > > acceptable, and we should try harder. > > > > > > > > I have not looked too deeply at it yet, but this is what I found from looking > > > > at a file in a randconfig build: > > > > > > > > Configuration: see https://pastebin.com/raw/R9erCwNj > > > > > > So this .config actually has KCSAN enabled. Do you still see the slowdown > > > with that disabled? > > > > Yes, enabling or disabling KCSAN seems to make no difference to > > compile speed in this config and source file, I still get the 12 seconds > > preprocessing time and 9MB file size with KCSAN disabled, possibly > > a few percent smaller/faster. I actually thought that CONFIG_FTRACE > > had a bigger impact, but disabling that also just reduces the time > > by a few percent rather than getting it down to the expected milliseconds. > > > > > Although not ideal, having a longer compiler time when > > > the compiler is being asked to perform instrumentation doesn't seem like a > > > show-stopper to me. > > > > I agree in general, but building an allyesconfig kernel is still an important > > use case that should not take twice as long after a small kernel change > > regardless of whether a new feature is used or not. (I have not actually > > compared the overall build speed for allmodconfig, as this takes a really > > long time at the moment) > > Note that an 'allyesconfig' selects KASAN and not KCSAN by default. > But I think that's not relevant, since KCSAN-specific code was removed > from ONCEs. In general though, it is entirely expected that we have a > bit longer compile times when we have the instrumentation passes > enabled. > > But as you pointed out, that's irrelevant, and the significant > overhead is from parsing and pre-processing. FWIW, we can probably > optimize Clang itself a bit: > https://github.com/ClangBuiltLinux/linux/issues/1032#issuecomment-633712667
Found that optimizing __unqual_scalar_typeof makes a noticeable difference. We could use C11's _Generic if the compiler supports it (and all supported versions of Clang certainly do).
Could you verify if the below patch improves compile-times for you? E.g. on fs/ocfs2/journal.c I was able to get ~40% compile-time speedup.
Thanks, -- Marco
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diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h index 5faf68eae204..a529fa263906 100644 --- a/include/linux/compiler_types.h +++ b/include/linux/compiler_types.h @@ -245,7 +245,9 @@ struct ftrace_likely_data { /* * __unqual_scalar_typeof(x) - Declare an unqualified scalar type, leaving * non-scalar types unchanged. - * + */ +#if defined(CONFIG_CC_IS_GCC) && CONFIG_GCC_VERSION < 40900 +/* * We build this out of a couple of helper macros in a vain attempt to * help you keep your lunch down while reading it. */ @@ -267,6 +269,24 @@ struct ftrace_likely_data { __pick_integer_type(x, int, \ __pick_integer_type(x, long, \ __pick_integer_type(x, long long, x)))))) +#else +/* + * If supported, prefer C11 _Generic for better compile-times. As above, 'char' + * is not type-compatible with 'signed char', and we define a separate case. + */ +#define __scalar_type_to_expr_cases(type) \ + type: (type)0, unsigned type: (unsigned type)0 + +#define __unqual_scalar_typeof(x) typeof( \ + _Generic((x), \ + __scalar_type_to_expr_cases(char), \ + signed char: (signed char)0, \ + __scalar_type_to_expr_cases(short), \ + __scalar_type_to_expr_cases(int), \ + __scalar_type_to_expr_cases(long), \ + __scalar_type_to_expr_cases(long long), \ + default: (x))) +#endif /* Is this type a native word size -- useful for atomic operations */ #define __native_word(t) \
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