Re: Question about barriers for ARM on tools/perf/

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Fri, May 08, 2015 at 03:48:20PM +0100, Will Deacon escreveu:
> On Fri, May 08, 2015 at 03:37:29PM +0100, Arnaldo Carvalho de Melo wrote:
> > Em Fri, May 08, 2015 at 04:25:13PM +0200, Peter Zijlstra escreveu:
> > > He wants to do smp refcounting, which needs atomic_inc() /
> > > atomic_inc_non_zero() / atomic_dec_return() etc..
> > 
> > Right, Will concentrated on what we use those barriers for right now in
> > tools/perf.
> > 
> > What I am doing right now is to expose what we use in perf to a wider
> > audience, i.e. code being developed in tools/, with the current intent
> > of implementing referece counting for multithreaded tools/perf/ tools,
> > right now only 'perf top', but there are patches floating to load a
> > perf.data file using as many CPUs as one would like, IIRC initially one
> > per available CPU.
> > 
> > I am using as a fallback the gcc intrinsics (), but I've heard I rather
> > should not use those, albeit they seemed to work well for x86_64 and
> > sparc64:
> 
> Do you know what the objection to the intrinsics was? I believe that
> the __sync versions are deprecated in favour of the C11-like __atomic
> flavours, so if that was all the objection was about then we could use
> one or the other depending on what the compiler supports.

Peter? Ingo?
 
> > One of my hopes for a byproduct was to take advantage of improvements
> > made to that code in the kernel, etc.
> > 
> > At least using the same API, i.e.  barrier(), mb(), rmb(), wmb(),
> > atomic_{inc,dec_and_test,read_init} I will, the whole shebang would be
> > even cooler.
> 
> Perhaps, but including atomic.h sounds pretty fragile to me. Sure, if we
> define the right set of macros we may get it to work today, but we could
> easily get subtle breakages as the kernel sources move forward and we might
> not easily notice/diagnose the failures in the perf tool.

Ok, that is a good argument not to share the same source code and
instead do what I am doing now, use it as the starting point, keep the
source code as much as possible the same, so that doing a:

  diff -u arch/$ARCH/include/asm/barrier.h tools/arch/$ARCH/include/asm/barrier.h

Would help in figuring out differences that may or may be desired, while
tracking what the kernel does would help keep the tools/ version in the
best possible shape.

This could even make it more likely that the kernel developers would
help having the best possible implementation in tools/ for that subset
of their work... :-)

- Arnaldo