From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Subject: Re: Performance overhead of get_cycles_sync Date: Tue, 11 Dec 2007 15:27:17 +0100 Message-ID: <20071211142717.GA15903@elte.hu> References: <475E8C8B.7070308@qumranet.com> <20071211133738.GA8150@elte.hu> <475E9A92.4030001@qumranet.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm-devel , Linux Kernel Mailing List To: dor.laor-atKUWr5tajBWk0Htik3J/w@public.gmane.org Return-path: Content-Disposition: inline In-Reply-To: <475E9A92.4030001-atKUWr5tajBWk0Htik3J/w@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: kvm-devel-bounces-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org Errors-To: kvm-devel-bounces-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org List-Id: kvm.vger.kernel.org * Dor Laor 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 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 Signed-off-by: Thomas Gleixner --- 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; ------------------------------------------------------------------------- SF.Net email is sponsored by: Check out the new SourceForge.net Marketplace. It's the best place to buy or sell services for just about anything Open Source. http://sourceforge.net/services/buy/index.php