From mboxrd@z Thu Jan 1 00:00:00 1970 From: Raghavendra K T Subject: Re: [PATCH RFC V11 15/18] kvm : Paravirtual ticketlocks support for linux guests running on KVM hypervisor Date: Wed, 24 Jul 2013 17:30:20 +0530 Message-ID: <51EFC1D4.9060800@linux.vnet.ibm.com> References: <20130722061631.24737.75508.sendpatchset@codeblue> <20130722062016.24737.54554.sendpatchset@codeblue> <20130723150748.GC6029@redhat.com> <51EFA24E.2060103@linux.vnet.ibm.com> <20130724103907.GF16400@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20130724103907.GF16400@redhat.com> Sender: linux-doc-owner@vger.kernel.org To: Gleb Natapov Cc: mingo@redhat.com, jeremy@goop.org, x86@kernel.org, konrad.wilk@oracle.com, hpa@zytor.com, pbonzini@redhat.com, linux-doc@vger.kernel.org, habanero@linux.vnet.ibm.com, xen-devel@lists.xensource.com, peterz@infradead.org, mtosatti@redhat.com, stefano.stabellini@eu.citrix.com, andi@firstfloor.org, attilio.rao@citrix.com, ouyang@cs.pitt.edu, gregkh@suse.de, agraf@suse.de, chegu_vinod@hp.com, torvalds@linux-foundation.org, avi.kivity@gmail.com, tglx@linutronix.de, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, riel@redhat.com, drjones@redhat.com, virtualization@lists.linux-foundation.org, srivatsa.vaddagiri@gmail.com List-Id: xen-devel@lists.xenproject.org On 07/24/2013 04:09 PM, Gleb Natapov wrote: > On Wed, Jul 24, 2013 at 03:15:50PM +0530, Raghavendra K T wrote: >> On 07/23/2013 08:37 PM, Gleb Natapov wrote: >>> On Mon, Jul 22, 2013 at 11:50:16AM +0530, Raghavendra K T wrote: >>>> +static void kvm_lock_spinning(struct arch_spinlock *lock, __ticket_t want) >> [...] >>>> + >>>> + /* >>>> + * halt until it's our turn and kicked. Note that we do safe halt >>>> + * for irq enabled case to avoid hang when lock info is overwritten >>>> + * in irq spinlock slowpath and no spurious interrupt occur to save us. >>>> + */ >>>> + if (arch_irqs_disabled_flags(flags)) >>>> + halt(); >>>> + else >>>> + safe_halt(); >>>> + >>>> +out: >>> So here now interrupts can be either disabled or enabled. Previous >>> version disabled interrupts here, so are we sure it is safe to have them >>> enabled at this point? I do not see any problem yet, will keep thinking. >> >> If we enable interrupt here, then >> >> >>>> + cpumask_clear_cpu(cpu, &waiting_cpus); >> >> and if we start serving lock for an interrupt that came here, >> cpumask clear and w->lock=null may not happen atomically. >> if irq spinlock does not take slow path we would have non null value >> for lock, but with no information in waitingcpu. >> >> I am still thinking what would be problem with that. >> > Exactly, for kicker waiting_cpus and w->lock updates are > non atomic anyway. > >>>> + w->lock = NULL; >>>> + local_irq_restore(flags); >>>> + spin_time_accum_blocked(start); >>>> +} >>>> +PV_CALLEE_SAVE_REGS_THUNK(kvm_lock_spinning); >>>> + >>>> +/* Kick vcpu waiting on @lock->head to reach value @ticket */ >>>> +static void kvm_unlock_kick(struct arch_spinlock *lock, __ticket_t ticket) >>>> +{ >>>> + int cpu; >>>> + >>>> + add_stats(RELEASED_SLOW, 1); >>>> + for_each_cpu(cpu, &waiting_cpus) { >>>> + const struct kvm_lock_waiting *w = &per_cpu(lock_waiting, cpu); >>>> + if (ACCESS_ONCE(w->lock) == lock && >>>> + ACCESS_ONCE(w->want) == ticket) { >>>> + add_stats(RELEASED_SLOW_KICKED, 1); >>>> + kvm_kick_cpu(cpu); >>> What about using NMI to wake sleepers? I think it was discussed, but >>> forgot why it was dismissed. >> >> I think I have missed that discussion. 'll go back and check. so >> what is the idea here? we can easily wake up the halted vcpus that >> have interrupt disabled? > We can of course. IIRC the objection was that NMI handling path is very > fragile and handling NMI on each wakeup will be more expensive then > waking up a guest without injecting an event, but it is still interesting > to see the numbers. > Haam, now I remember, We had tried request based mechanism. (new request like REQ_UNHALT) and process that. It had worked, but had some complex hacks in vcpu_enter_guest to avoid guest hang in case of request cleared. So had left it there.. https://lkml.org/lkml/2012/4/30/67 But I do not remember performance impact though.