Re: Performance overhead of get_cycles_sync

Dor Laor
2007-12-11T15:03:50Z

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?
>
It works, actually I already commented it out.

Acked-by: Dor Laor <dor.laor@qumranet.com>

But this CPUID was present in v2.6.23 too, so why did it only show up in
> 2.6.24-rc for you?
>
>
I tried to figure out but all the code movements for i386 go in the way.
In the previous email I reported to Andi that Fedora kernel 2.6.23-8 did
not suffer from it.
Thanks for the ultra fast reply :)
Dor
> 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;
>
>