Re: [PATCH 3/4] perf tool: Fix build when sysconf doesn't support cache line size

[Chris Phlipot]

On 07/04/2016 05:26 PM, Arnaldo Carvalho de Melo wrote:
> Em Mon, Jul 04, 2016 at 05:19:20PM -0700, Chris Phlipot escreveu:
> > On 07/04/2016 03:48 PM, Arnaldo Carvalho de Melo wrote:
> > > Em Thu, Jun 30, 2016 at 10:12:34PM -0700, Chris Phlipot escreveu:
> > > > Enable perf to build on libc implementations where sysconf() doesn't
> > > > support _SC_LEVEL1_DCACHE_LINESIZE as a parameter.
> > > >
> > > > For example, the Bionic implementation does not support this as a
> > > > paremter. Older versions of Bionic will throw an error when this is passed
> > > > in as a parameter, and more recent versions will just return 0 as the
> > > > cache line size.
> > > >
> > > > Signed-off-by: Chris Phlipot <cphlipot0@xxxxxxxxx>
> > > > ---
> > > >    tools/perf/perf.c | 4 ++++
> > > >    1 file changed, 4 insertions(+)
> > > >
> > > > diff --git a/tools/perf/perf.c b/tools/perf/perf.c
> > > > index 8f21922..113ca5b 100644
> > > > --- a/tools/perf/perf.c
> > > > +++ b/tools/perf/perf.c
> > > > @@ -509,7 +509,11 @@ int main(int argc, const char **argv)
> > > >
> > > >    	/* The page_size is placed in util object. */
> > > >    	page_size = sysconf(_SC_PAGE_SIZE);
> > > > +#ifdef _SC_LEVEL1_DCACHE_LINESIZE
> > > >    	cacheline_size = sysconf(_SC_LEVEL1_DCACHE_LINESIZE);
> > > > +#else
> > > > +	cacheline_size = 0;
> > > > +#endif
> > >
> > > Couldn't we instead fallback to:
> > >
> > > sysfs__read_int("devices/system/cpu/cpu0/cache/index0/coherency_line_size", &cacheline_size)
> > >
> > > ?
> >
> > I agree that in general this would be a better fallback, but in  all Android
> > images I have tested so far, "devices/system/cpu/cpu0/cache" does not exist.
> > I know not know of a good way to retrieve cache line size in this case.
> >
> > I would be ok with attempting to get cacheline size using using the
> > following methods, unless you have other ideas:
> >
> > 1. attempt to use sysconf(_SC_LEVEL1_DCACHE_LINESIZE)
> > 2. attempt to use
> > sysfs__read_int("devices/system/cpu/cpu0/cache/index0/coherency_line_size",
> > &cacheline_size)
> > 3. set to zero if both of the above fail.
>
> Ok, but perhaps we should have some sort of warning in places using
> this?
>
> - Arnaldo

Such as printing a warning when cacheline_size is set to zero, or simply 
adding comments to the code in areas where cacheline_size is used?

-Chris

> > > >    	if (sysctl__read_int("kernel/perf_event_max_stack", &value) == 0)
> > > >    		sysctl_perf_event_max_stack = value;
> > > > --
> > > > 2.7.4