Messages in this thread Patch in this message | | | Date | Tue, 11 Dec 2007 15:27:17 +0100 | From | Ingo Molnar <> | Subject | Re: Performance overhead of get_cycles_sync |
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* 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;
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