From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gleb Natapov Subject: Re: [PATCH RFC V10 15/18] kvm : Paravirtual ticketlocks support for linux guests running on KVM hypervisor Date: Wed, 17 Jul 2013 17:44:09 +0300 Message-ID: <20130717144409.GD13732@redhat.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> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <51E6A66D.7090407@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: Raghavendra K T 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 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. > >How often this will happens anyway. > > > > I know NMIs occur frequently with watchdogs. or used by sysrq-trigger > etc.. But I am not an expert how frequent it is otherwise. But even > then if they do not use spinlock, we have no problem as already pointed. > > I can measure with debugfs counter how often it happens. > When you run perf you will see a lot of NMIs, but those should not take any locks. -- Gleb.