Messages in this thread | | | Date | Fri, 8 May 2015 11:57:01 -0300 | From | Arnaldo Carvalho de Melo <> | Subject | Re: Question about barriers for ARM on tools/perf/ |
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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
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