From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754265Ab2GSJRx (ORCPT ); Thu, 19 Jul 2012 05:17:53 -0400 Received: from e39.co.us.ibm.com ([32.97.110.160]:40287 "EHLO e39.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754000Ab2GSJRu (ORCPT ); Thu, 19 Jul 2012 05:17:50 -0400 Date: Thu, 19 Jul 2012 14:45:31 +0530 From: Raghavendra K T To: Christian Borntraeger , linux390@de.ibm.com, "H. Peter Anvin" , Thomas Gleixner , Marcelo Tosatti , Ingo Molnar , Avi Kivity , Rik van Riel , KVM Cc: S390 , Carsten Otte , chegu vinod , "Andrew M. Theurer" , LKML , X86 , Gleb Natapov , Srivatsa Vaddagiri , Joerg Roedel Subject: [RESEND PATCH RFC V3 0/3] kvm: Improving directed yield in PLE handler Message-ID: <20120719091531.GA29888@linux.vnet.ibm.com> Reply-To: Raghavendra K T References: <20120712191712.30440.68944.sendpatchset@codeblue> <4FFF2440.40807@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <4FFF2440.40807@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12071909-4242-0000-0000-0000025FF645 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Raghavendra K T Currently, on a large vcpu guests, there is a high probability of yielding to the same vcpu who had recently done a pause-loop exit or cpu relax intercepted. Such a yield can lead to the vcpu spinning again and hence degrade the performance. The patchset keeps track of the pause loop exit/cpu relax interception and gives chance to a vcpu which: (a) Has not done pause loop exit or cpu relax intercepted at all (probably he is preempted lock-holder) (b) Was skipped in last iteration because it did pause loop exit or cpu relax intercepted, and probably has become eligible now (next eligible lock holder) Signed-off-by: Raghavendra K T --- V2 was: Reviewed-by: Rik van Riel Changelog: Added comment on locking as suggested by Avi include/linux/kvm_host.h | 5 +++++ virt/kvm/kvm_main.c | 42 ++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 47 insertions(+), 0 deletions(-) diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index 34ce296..952427d 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -923,6 +923,11 @@ static inline void kvm_vcpu_set_dy_eligible(struct kvm_vcpu *vcpu, bool val) { } +static inline bool kvm_vcpu_eligible_for_directed_yield(struct kvm_vcpu *vcpu) +{ + return true; +} + #endif /* CONFIG_HAVE_KVM_CPU_RELAX_INTERCEPT */ #endif diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 3d6ffc8..8fda756 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -1571,6 +1571,43 @@ bool kvm_vcpu_yield_to(struct kvm_vcpu *target) } EXPORT_SYMBOL_GPL(kvm_vcpu_yield_to); +#ifdef CONFIG_HAVE_KVM_CPU_RELAX_INTERCEPT +/* + * Helper that checks whether a VCPU is eligible for directed yield. + * Most eligible candidate to yield is decided by following heuristics: + * + * (a) VCPU which has not done pl-exit or cpu relax intercepted recently + * (preempted lock holder), indicated by @in_spin_loop. + * Set at the beiginning and cleared at the end of interception/PLE handler. + * + * (b) VCPU which has done pl-exit/ cpu relax intercepted but did not get + * chance last time (mostly it has become eligible now since we have probably + * yielded to lockholder in last iteration. This is done by toggling + * @dy_eligible each time a VCPU checked for eligibility.) + * + * Yielding to a recently pl-exited/cpu relax intercepted VCPU before yielding + * to preempted lock-holder could result in wrong VCPU selection and CPU + * burning. Giving priority for a potential lock-holder increases lock + * progress. + * + * Since algorithm is based on heuristics, accessing another VCPU data without + * locking does not harm. It may result in trying to yield to same VCPU, fail + * and continue with next VCPU and so on. + */ +bool kvm_vcpu_eligible_for_directed_yield(struct kvm_vcpu *vcpu) +{ + bool eligible; + + eligible = !vcpu->spin_loop.in_spin_loop || + (vcpu->spin_loop.in_spin_loop && + vcpu->spin_loop.dy_eligible); + + if (vcpu->spin_loop.in_spin_loop) + kvm_vcpu_set_dy_eligible(vcpu, !vcpu->spin_loop.dy_eligible); + + return eligible; +} +#endif void kvm_vcpu_on_spin(struct kvm_vcpu *me) { struct kvm *kvm = me->kvm; @@ -1599,6 +1636,8 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me) continue; if (waitqueue_active(&vcpu->wq)) continue; + if (!kvm_vcpu_eligible_for_directed_yield(vcpu)) + continue; if (kvm_vcpu_yield_to(vcpu)) { kvm->last_boosted_vcpu = i; yielded = 1; @@ -1607,6 +1646,9 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me) } } kvm_vcpu_set_in_spin_loop(me, false); + + /* Ensure vcpu is not eligible during next spinloop */ + kvm_vcpu_set_dy_eligible(me, false); } EXPORT_SYMBOL_GPL(kvm_vcpu_on_spin);