Re: cpuinfo_x86 and apicid

Stephane Eranian
2006-07-06T22:34:01Z

Suresh,

On Thu, Jul 06, 2006 at 02:06:13PM -0700, Siddha, Suresh B wrote:
> On Thu, Jul 06, 2006 at 01:00:31PM -0700, Stephane Eranian wrote:
> > On Thu, Jul 06, 2006 at 09:19:30AM -0700, Siddha, Suresh B wrote:
> > > On Thu, Jul 06, 2006 at 08:01:18AM -0700, Stephane Eranian wrote:
> > > > Hello,
> > > >
> > > >
> > > > In the context of the perfmon2 subsystem for processor with HyperThreading,
> > > > we need to know on which thread we are currently running. This comes from
> > > > the fact that the performance counters are shared between the two threads.
> > > >
> > > > We use the thread id (smt_id) because we split the counters in half
> > > > between the two threads such that two threads on the same core can run
> > > > with monitoring on. We are currently computing the smt_id from the
> > > > apicid as returned by a CPUID instruction. This is not very efficient.
> > > >
> > > > I looked through the i386 code and could not find a function nor
> > > > structure that would return this smt_id. In the cpuinfo_x86 structure
> > > > there is an apicid field that looks good, yet it does not seem to be
> > > > initialized nor used.
> > > >
> > > > Is cpuinfo_x86->apicid field obsolete?
> > > > If so, what is replacing it?
> > >
> > > In i386, it is getting initialized in generic_identify() in common.c and
> > > it is getting used for example in intel_cacheinfo.c
> > >
> > Well, yes this is exactly what I want, except that it is not compiled for x86_64
> > so on a HT Xeon in 64-bit I don't get it. Why is that?
>
> on x86_64, it is getting initialized in identify_cpu() and further
> intel_cacheinfo.c gets linked in both i386 and x86_64.
>
Ah, yes I missed that. It works there two. I had something wrong
about how I accessed cpu_data. I am used to the elegant way we
do it on IA-64 but on x86_64 you have to index the cpu_data[]
array with smp_processor_id(). I was pointing to cpu_data[0]
on all processors.

For what I need, I can do cpuinfo_x86->apicid & 0x3 to identify
which thread is running. I can now remove some more code in perfmon2.

Thanks for your help.

--
-Stephane