From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752190Ab3GXJie (ORCPT ); Wed, 24 Jul 2013 05:38:34 -0400 Received: from e28smtp02.in.ibm.com ([122.248.162.2]:38423 "EHLO e28smtp02.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751251Ab3GXJic (ORCPT ); Wed, 24 Jul 2013 05:38:32 -0400 Message-ID: <51EFA24E.2060103@linux.vnet.ibm.com> Date: Wed, 24 Jul 2013 15:15:50 +0530 From: Raghavendra K T Organization: IBM User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:16.0) Gecko/20121029 Thunderbird/16.0.2 MIME-Version: 1.0 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 Subject: Re: [PATCH RFC V11 15/18] kvm : Paravirtual ticketlocks support for linux guests running on KVM hypervisor References: <20130722061631.24737.75508.sendpatchset@codeblue> <20130722062016.24737.54554.sendpatchset@codeblue> <20130723150748.GC6029@redhat.com> In-Reply-To: <20130723150748.GC6029@redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-MML: No X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13072409-5816-0000-0000-0000091124B6 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. >> + 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?