Re: [RFC] perf: fix building for ARCv1

[Alexey Brodkin]

Hi Vineet,

Looks like this time atomics are a must. And that really sucks!

See these commits that introduce usage of atomic_xxx() all around the perf and tools it uses:
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=f812d3045c2385ac16237e68b156859c4005526e
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=d3a7c489c7fd2463e3b2c3a2179c7be879dd9cb4
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=7143849a5d6a5c623d81790d92f0033507c5b14f
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=59a51c1dc9fbb3fb4af928b852d7b35df83edd74
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=e1ed3a5b87ed6759e16ec93f16aae83d2cc77ca2

and that's the one that introduced usage of the following generic gcc's atomics
(__sync_add_and_fetch/__sync_sub_and_fetch):
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=da6d8567512df11e0473b710c07de87efde5709c

So the best we may do is to implement detection of atomics in the toolchain and if there's no atomics hard stop with
perf building.

-Alexey

On Sat, 2015-10-17 at 14:19 +0000, Vineet Gupta wrote:
> On Saturday 17 October 2015 07:06 PM, Alexey Brodkin wrote:
> > Perf uses atomic options and so it is required to have atomics enabled
> > in toolchain.
> > 
> > In case of ARC atomics are enabled by default for ARCv2 but disabled for
> > ARCv1. Now we explicitly enable atomics for either ARC achitecture
> > version so perf could be successfully built.
> > 
> > Currently on attempt to build perf for ARCv1 you'll see tons of:
> > ----------------->8-----------------
> > undefined reference to `__sync_add_and_fetch_4'
> > ----------------->8-----------------
> > 
> > Still note if ARCv1 CPU is configured without LL/SC perf will crash on
> > execution once "llock" instruction is attempted to be executed.
> 
> Ok this fixes ARCompact - assuming it will have LL/SC. We do have old SoCs w/o
> that support.
> So what we are saying is that any arch (or a configuration thereof) which doesn't
> support atomic r-m-w can't even build perf now - that sucks !
> 
> A better way would be to do feature test for __sync_xyz and make atomic_xxx
> wrappers call __sync_xyz) vs. an empty stub.
> So atleast such arches can build and do "some" perf work !
> 
> -Vineet
> 
> > Cc: Vineet Gupta <vgupta@xxxxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> > Signed-off-by: Alexey Brodkin <abrodkin@xxxxxxxxxxxx>
> > ---
> >  tools/perf/config/Makefile | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/tools/perf/config/Makefile b/tools/perf/config/Makefile
> > index 38a0853..dc7c0a8 100644
> > --- a/tools/perf/config/Makefile
> > +++ b/tools/perf/config/Makefile
> > @@ -47,6 +47,11 @@ ifeq ($(ARCH),arm64)
> >    LIBUNWIND_LIBS = -lunwind -lunwind-aarch64
> >  endif
> >  
> > +# Additional ARCH settings for ARC
> > +ifeq ($(ARCH),arc)
> > +  CFLAGS += -matomic
> > +endif
> > +
> >  ifeq ($(NO_PERF_REGS),0)
> >    $(call detected,CONFIG_PERF_REGS)
> >  endif
> 
>