From mboxrd@z Thu Jan 1 00:00:00 1970 From: Raghavendra K T Subject: Re: [PATCH RFC V10 15/18] kvm : Paravirtual ticketlocks support for linux guests running on KVM hypervisor Date: Wed, 17 Jul 2013 20:50:13 +0530 Message-ID: <51E6B62D.50106@linux.vnet.ibm.com> References: <20130715103648.GN11772@redhat.com> <51E4C011.4060803@linux.vnet.ibm.com> <20130716060215.GE11772@redhat.com> <51E5941B.3090300@linux.vnet.ibm.com> <20130717093420.GU11772@redhat.com> <51E66C71.6020605@linux.vnet.ibm.com> <20130717124511.GW11772@redhat.com> <51E69429.7040309@linux.vnet.ibm.com> <20130717132503.GA13732@redhat.com> <51E6A66D.7090407@linux.vnet.ibm.com> <20130717144409.GD13732@redhat.com> <51E6B057.5080905@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <51E6B057.5080905@linux.vnet.ibm.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Gleb Natapov Cc: jeremy@goop.org, kvm@vger.kernel.org, linux-doc@vger.kernel.org, peterz@infradead.org, drjones@redhat.com, virtualization@lists.linux-foundation.org, andi@firstfloor.org, hpa@zytor.com, stefano.stabellini@eu.citrix.com, xen-devel@lists.xensource.com, x86@kernel.org, mingo@redhat.com, habanero@linux.vnet.ibm.com, riel@redhat.com, konrad.wilk@oracle.com, ouyang@cs.pitt.edu, avi.kivity@gmail.com, tglx@linutronix.de, chegu_vinod@hp.com, linux-kernel@vger.kernel.org, srivatsa.vaddagiri@gmail.com, pbonzini@redhat.com, torvalds@linux-foundation.org List-Id: xen-devel@lists.xenproject.org On 07/17/2013 08:25 PM, Raghavendra K T wrote: > On 07/17/2013 08:14 PM, Gleb Natapov wrote: >> On Wed, Jul 17, 2013 at 07:43:01PM +0530, Raghavendra K T wrote: >>> On 07/17/2013 06:55 PM, Gleb Natapov wrote: >>>> On Wed, Jul 17, 2013 at 06:25:05PM +0530, Raghavendra K T wrote: >>>>> On 07/17/2013 06:15 PM, Gleb Natapov wrote: >>>>>> On Wed, Jul 17, 2013 at 03:35:37PM +0530, Raghavendra K T wrote: >>>>>>>>> Instead of halt we started with a sleep hypercall in those >>>>>>>>> versions. Changed to halt() once Avi suggested to reuse >>>>>>>>> existing sleep. >>>>>>>>> >>>>>>>>> If we use older hypercall with few changes like below: >>>>>>>>> >>>>>>>>> kvm_pv_wait_for_kick_op(flags, vcpu, w->lock ) >>>>>>>>> { >>>>>>>>> // a0 reserved for flags >>>>>>>>> if (!w->lock) >>>>>>>>> return; >>>>>>>>> DEFINE_WAIT >>>>>>>>> ... >>>>>>>>> end_wait >>>>>>>>> } >>>>>>>>> >>>>>>>> How would this help if NMI takes lock in critical section. The >>>>>>>> thing >>>>>>>> that may happen is that lock_waiting->want may have NMI lock >>>>>>>> value, but >>>>>>>> lock_waiting->lock will point to non NMI lock. Setting of want >>>>>>>> and lock >>>>>>>> have to be atomic. >>>>>>> >>>>>>> True. so we are here >>>>>>> >>>>>>> non NMI lock(a) >>>>>>> w->lock = NULL; >>>>>>> smp_wmb(); >>>>>>> w->want = want; >>>>>>> NMI >>>>>>> <--------------------- >>>>>>> NMI lock(b) >>>>>>> w->lock = NULL; >>>>>>> smp_wmb(); >>>>>>> w->want = want; >>>>>>> smp_wmb(); >>>>>>> w->lock = lock; >>>>>>> ----------------------> >>>>>>> smp_wmb(); >>>>>>> w->lock = lock; >>>>>>> >>>>>>> so how about fixing like this? >>>>>>> >>>>>>> again: >>>>>>> w->lock = NULL; >>>>>>> smp_wmb(); >>>>>>> w->want = want; >>>>>>> smp_wmb(); >>>>>>> w->lock = lock; >>>>>>> >>>>>>> if (!lock || w->want != want) goto again; >>>>>>> >>>>>> NMI can happen after the if() but before halt and the same situation >>>>>> we are trying to prevent with IRQs will occur. >>>>> >>>>> True, we can not fix that. I thought to fix the inconsistency of >>>>> lock,want pair. >>>>> But NMI could happen after the first OR condition also. >>>>> /me thinks again >>>>> >>>> lock_spinning() can check that it is called in nmi context and bail >>>> out. >>> >>> Good point. >>> I think we can check for even irq context and bailout so that in irq >>> context we continue spinning instead of slowpath. no ? >>> >> That will happen much more often and irq context is no a problem anyway. >> > > Yes. It is not a problem. But my idea was to not to enter slowpath lock > during irq processing. Do you think that is a good idea? > > I 'll now experiment how often we enter slowpath in irq context. > With dbench 1.5x run, on my 32cpu / 16core sandybridge, I saw around 10 spinlock slowpath entered from the irq context.