Re: cpuinfo_x86 and apicid

[linux-os (Dick Johnson)]

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/
_



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Thank you./*
 *  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;
}