Re: [kvm-devel] Performance overhead of get_cycles_sync

Joerg Roedel
2007-12-11T21:30:47Z

On Tue, Dec 11, 2007 at 03:27:17PM +0100, Ingo Molnar wrote:
>
> * Dor Laor <dor.laor@gmail.com> wrote:
>
> > Here [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);
>
> The patch below should resolve this - could you please test and Ack it?
> But this CPUID was present in v2.6.23 too, so why did it only show up in
> 2.6.24-rc for you?
>
> Ingo
>
> -------------->
> Subject: x86: fix get_cycles_sync() overhead
> From: Ingo Molnar <mingo@elte.hu>
>
> get_cycles_sync() is causing massive overhead in KVM networking:
>
> http://lkml.org/lkml/2007/12/11/54
>
> remove the explicit CPUID serialization - it causes VM exits and is
> pointless: we care about GTOD coherency but that goes to user-space
> via a syscall, and syscalls are serialization points anyway.
>
> Signed-off-by: Ingo Molnar <mingo@elte.hu>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> include/asm-x86/tsc.h | 12 ++++++------
> 1 file changed, 6 insertions(+), 6 deletions(-)
>
> Index: linux-x86.q/include/asm-x86/tsc.h
> ===================================================================
> --- linux-x86.q.orig/include/asm-x86/tsc.h
> +++ linux-x86.q/include/asm-x86/tsc.h
> @@ -39,8 +39,8 @@ static __always_inline cycles_t get_cycl
> unsigned eax, edx;
>
> /*
> - * Use RDTSCP if possible; it is guaranteed to be synchronous
> - * and doesn't cause a VMEXIT on Hypervisors
> + * 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)),
> @@ -50,11 +50,11 @@ static __always_inline cycles_t get_cycl
> return ret;
>
> /*
> - * Don't do an additional sync on CPUs where we know
> - * RDTSC is already synchronous:
> + * Use RDTSC on other CPUs. This might not be fully synchronous,
> + * but it's not a problem: the only coherency we care about is
> + * the GTOD output to user-space, and syscalls are synchronization
> + * points anyway:
> */
> - alternative_io("cpuid", ASM_NOP2, X86_FEATURE_SYNC_RDTSC,
> - "=a" (eax), "0" (1) : "ebx","ecx","edx","memory");
> rdtscll(ret);
>
> return ret;

I don't think this is a good idea. I discussed exactly this item with
Andi Kleen a while ago and afair the serializing instruction was
necessary to fix a backwards walking gettimeofday() on some K8
revisions. Andi Kleen can tell more details, I added him to the CC list.

Joerg

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