Re: cpuinfo_x86 and apicid

linux-os (Dick Johnson)
2006-07-06T15:28:28Z

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On Thu, 6 Jul 2006, 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?
>
> Thanks.
>
> --
> -Stephane
> -

Does the attached file help? It is supposed to tell which CPU
execution is occurring on. This might be modified to fill your
needs.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.4 on an i686 machine (5592.88 BogoMips).
New book: http://www.AbominableFirebug.com/
_
\x1a\x04

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[-- Attachment #2: whichcpu.c --]
[-- Type: TEXT/PLAIN, Size: 874 bytes --]

/*
* This program tells you what CPU your process is executing on.
* If you comment-out the usleep(10000), so you are eating all the
* CPU time in your time-slice, and you execute this from two tasks,
* you will note that each task gets its own CPU if you have two
* CPUs.
* If you leave in the usleep(), you will note that both tasks may
* use the same CPU. This happens if, and only if, they are not
* executing at the same time.
*/

#include <stdio.h>
int main(void);

int main()
{
int cpu, prv;
prv = -1;
for(;;)
{
__asm__ __volatile__ (
"xorl %%eax,%%eax\n"
"str %%ax\n"
"subl $0x60, %%eax\n"
"shrl $0x05, %%eax\n"
: "=a"(cpu));
if(cpu != prv)
{
prv = cpu;
printf("CPU %d\n", cpu);
}
usleep(10000);
}
return 0;
}