From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested Date: Thu, 19 Jul 2018 05:55:15 -0700 Message-ID: <20180719125515.GS12945@linux.vnet.ibm.com> References: <1531815548.19223.23.camel@infradead.org> <20180717125653.GH12945@linux.vnet.ibm.com> <20180718153628.GA24359@linux.vnet.ibm.com> <1531929711.3414.29.camel@infradead.org> <20180718163712.GB12945@linux.vnet.ibm.com> <1531942865.3414.35.camel@infradead.org> <20180718201700.GN12945@linux.vnet.ibm.com> <86d12d1a-46dd-d82a-96c2-e842ac5a2a6c@de.ibm.com> <1531984833.12620.16.camel@infradead.org> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Cc: David Woodhouse , Peter Zijlstra , mhillenb@amazon.de, linux-kernel , kvm , Frederic Weisbecker To: Christian Borntraeger Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On Thu, Jul 19, 2018 at 12:23:34PM +0200, Christian Borntraeger wrote: > > > On 07/19/2018 09:20 AM, David Woodhouse wrote: > > On Thu, 2018-07-19 at 08:45 +0200, Christian Borntraeger wrote: > >> > >>> My thought would be something like this: > >>>   > >>>        if (context_tracking_cpu_is_enabled()) > >>>                rcu_kvm_enter(); > >>>        else > >>>                rcu_virt_note_context_switch(smp_processor_id()); > >> > >> In the past we needed that (when we introduced that). At least with every > >> host interrupt we called this making an rcu event at least every HZ. > >> Will the changes in need_resched make this part unnecessary? > > > > Yes, the change in need_resched() should make this part unnecessary. > > Unless your architecture's version of the vcpu_run() loop just loops > > forever even when TIF_NEED_RESCHED is set? :) > > Very early versions did that. The SIE instruction is interruptible > so you can continue to run the guest by simply returning from an host > interrupt. All sane versions of KVM on s390 now make sure to make a > short trip into C after a host interrupt. There we check for > need_resched signals and machine checks so we are good. OK, thank you all! I will drop the two patches that add the rcu_kvm_enter() and rcu_kvm_exit() calls. Two less things to worry about! ;-) Thanx, Paul > > I'm not sure about the context tracking condition in the code snippet > > cited above, though. I think that's what caused my problem in the first > > place — I have CONTEXT_TRACKING_FORCE && !NO_HZ_FULL. So in 4.15, that > > means rcu_user_enter() did nothing and rcu_virt_note_context_switch() > > wasn't called. Hence the observed stalls. > > > > Should rcu_user_enter() itself be conditional on CONTEXT_TRACKING not > > NO_HZ_FULL?  > >