Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932348Ab2K0SMm (ORCPT ); Tue, 27 Nov 2012 13:12:42 -0500 Received: from mail-vb0-f46.google.com ([209.85.212.46]:45404 "EHLO mail-vb0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756286Ab2K0SMl (ORCPT ); Tue, 27 Nov 2012 13:12:41 -0500 MIME-Version: 1.0 In-Reply-To: <20121127174737.GN12969@redhat.com> References: <1353993325.14050.49.camel@ThinkPad-T5421.cn.ibm.com> <20121127154455.GK12969@redhat.com> <20121127163910.GL12969@redhat.com> <20121127170026.GM12969@redhat.com> <20121127174737.GN12969@redhat.com> Date: Tue, 27 Nov 2012 19:12:40 +0100 Message-ID: Subject: Re: [RFC PATCH] Fix abnormal rcu dynticks_nesting values related to async page fault From: Frederic Weisbecker To: Gleb Natapov Cc: Li Zhong , linux-next list , LKML , paulmck@linux.vnet.ibm.com, sasha.levin@oracle.com, avi@redhat.com Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2147 Lines: 48 2012/11/27 Gleb Natapov : > On Tue, Nov 27, 2012 at 06:30:32PM +0100, Frederic Weisbecker wrote: >> 2012/11/27 Gleb Natapov : >> > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c >> > index 4180a87..636800d 100644 >> > --- a/arch/x86/kernel/kvm.c >> > +++ b/arch/x86/kernel/kvm.c >> > @@ -113,7 +113,7 @@ void kvm_async_pf_task_wait(u32 token) >> > int cpu, idle; >> > >> > cpu = get_cpu(); >> > - idle = idle_cpu(cpu); >> > + idle = is_idle_task(current); >> >> I suggest this part goes to a standalone patch. >> >> > put_cpu(); >> > >> > spin_lock(&b->lock); >> > @@ -247,10 +247,7 @@ do_async_page_fault(struct pt_regs *regs, unsigned long error_code) >> > break; >> > case KVM_PV_REASON_PAGE_NOT_PRESENT: >> > /* page is swapped out by the host. */ >> > - rcu_irq_enter(); >> > - exit_idle(); >> > kvm_async_pf_task_wait((u32)read_cr2()); >> > - rcu_irq_exit(); >> >> Hmm, we still need those above around. I believe we just need to add >> rcu_user_exit() in the beginning of that case. > The exception may happen in kernel space too. Is calling rcu_user_exit() > still OK? Also why calling exit_idle() if we are not exiting idle? Yeah, rcu_user_exit() takes care of that. And exit_idle() also checks we are really idle before firing the notifier. Now we should probably call back enter_idle() before resuming idle if needed. We disable irqs before calling enter_idle(). And exit_idle() is called from irqs. This way we ensure it's either called before we called local_irq_disable() or while the CPU is halt(). This provides the guarantee that enter_idle() is always called before the CPU goes to sleep. The fact we call exit_idle() from an exception in idle breaks this guarantee. But that's another issue. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/