Re: Performance overhead of get_cycles_sync

[Dor Laor <dor.laor-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Ingo Molnar wrote:
>
> * Dor Laor <dor.laor-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>
> > Hi Ingo, Thomas,
> >
> > In the latest kernel (2.6.24-rc3) I noticed a drastic performance
> > decrease for KVM networking. The reason is many vmexit (exit reason is
> > cpuid instruction) caused by calls to gettimeofday that uses tsc
> > sourceclock. read_tsc calls get_cycles_sync which might call cpuid in
> > order to serialize the cpu.
> >
> > Can you explain why the cpu needs to be serialized for every gettime
> > call? Do we need to be that accurate? (It will also slightly improve
> > physical hosts). I believe you have a reason and the answer is yes. In
> > that case can you replace the serializing instruction with an
> > instruction that does not trigger vmexit? Maybe use 'ltr' for example?
>
> hm, where exactly does it call CPUID?
>
>         Ingo
>
Here, commented out [include/asm-x86/tsc.h]:
/* Like get_cycles, but make sure the CPU is synchronized. */
static __always_inline cycles_t get_cycles_sync(void)
{
    unsigned long long ret;
    unsigned eax, edx;

    /*
       * Use RDTSCP if possible; it is guaranteed to be synchronous
      * and doesn't cause a VMEXIT on Hypervisors
     */
    alternative_io(ASM_NOP3, ".byte 0x0f,0x01,0xf9", X86_FEATURE_RDTSCP,
               ASM_OUTPUT2("=a" (eax), "=d" (edx)),
               "a" (0U), "d" (0U) : "ecx", "memory");
    ret = (((unsigned long long)edx) << 32) | ((unsigned long long)eax);
    if (ret)
        return ret;

    /*
     * Don't do an additional sync on CPUs where we know
     * RDTSC is already synchronous:
     */
//    alternative_io("cpuid", ASM_NOP2, X86_FEATURE_SYNC_RDTSC,
//              "=a" (eax), "0" (1) : "ebx","ecx","edx","memory");
    rdtscll(ret);

    return ret;
}


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