Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756544Ab2K0QsX (ORCPT ); Tue, 27 Nov 2012 11:48:23 -0500 Received: from mail-vc0-f174.google.com ([209.85.220.174]:60137 "EHLO mail-vc0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756123Ab2K0QsU (ORCPT ); Tue, 27 Nov 2012 11:48:20 -0500 MIME-Version: 1.0 In-Reply-To: <20121127161925.GG2474@linux.vnet.ibm.com> References: <1353993325.14050.49.camel@ThinkPad-T5421.cn.ibm.com> <20121127154455.GK12969@redhat.com> <20121127161925.GG2474@linux.vnet.ibm.com> Date: Tue, 27 Nov 2012 17:48:18 +0100 Message-ID: Subject: Re: [RFC PATCH] Fix abnormal rcu dynticks_nesting values related to async page fault From: Frederic Weisbecker To: paulmck@linux.vnet.ibm.com Cc: Gleb Natapov , Li Zhong , linux-next list , LKML , 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: 1514 Lines: 33 2012/11/27 Paul E. McKenney : > On Tue, Nov 27, 2012 at 04:56:30PM +0100, Frederic Weisbecker wrote: >> 2012/11/27 Gleb Natapov : >> > For KVM_PV_REASON_PAGE_NOT_PRESENT it behaves like an exception. >> >> Ok. >> There seem to be a bug in kvm_async_pf_task_wait(). Using >> idle_cpu(cpu) to find out if the current task is the idle task may not >> work if there is pending wake up. Me may schedule another task but >> when that other task sleeps later we can't schedule back to idle until >> the fault is completed. >> >> The right way is to use is_idle_task(current) > > Agreed. But if you instead need to figure out whether or not RCU believes > that the CPU is idle, use rcu_is_cpu_idle(). You need to figure out > whether or not RCU believes that the CPU is idle if you are trying to > play fast and loose with rcu_irq_enter() and rcu_irq_exit(). Right. I was referring to another problem though. They don't want to schedule on the following cases: 1) preemption disabled, of course. 3) we are in the idle task. Because if we want to sleep in order to wait for the page fault completion, we need to be able to schedule the idle task. But we are already the idle task. Now idle_cpu(cpu) is the wrong check for that. -- 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/