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: Wed, 18 Jul 2018 20:11:52 -0700 Message-ID: <20180719031152.GR12945@linux.vnet.ibm.com> References: <1531168538.26547.5.camel@infradead.org> <20180709204248.GF3593@linux.vnet.ibm.com> <1531169145.26547.8.camel@infradead.org> <20180709210532.GH3593@linux.vnet.ibm.com> <20180709220823.GA18045@linux.vnet.ibm.com> <1531319025.8759.57.camel@infradead.org> <20180711144303.GQ3593@linux.vnet.ibm.com> <20180711164952.GA29994@linux.vnet.ibm.com> <20180719003205.GB5595@lerouge> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: David Woodhouse , Peter Zijlstra , mhillenb@amazon.de, linux-kernel , kvm To: Frederic Weisbecker Return-path: Content-Disposition: inline In-Reply-To: <20180719003205.GB5595@lerouge> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On Thu, Jul 19, 2018 at 02:32:06AM +0200, Frederic Weisbecker wrote: > On Wed, Jul 11, 2018 at 06:03:42PM +0100, David Woodhouse wrote: > > On Wed, 2018-07-11 at 09:49 -0700, Paul E. McKenney wrote: > > > And here is an updated v4.15 patch with Marius's Reported-by and David's > > > fix to my lost exclamation point. > > > > Thanks. Are you sending the original version of that to Linus? It'd be > > useful to have the commit ID so that we can watch for it landing, and > > chase this one up to Greg. > > > > As discussed on IRC, this patch reduces synchronize_sched() latency for > > us from ~4600s to ~160ms, which is nice. > > > > However, it isn't going to be sufficient in the NO_HZ_FULL case. For > > that you want a patch like the one below, which happily reduces the > > latency in our (!NO_HZ_FULL) case still further to ~40ms. > > That is interesting. As I replied to Paul, we are already calling > rcu_user_enter/exit() on guest_enter/exit_irqsoff(). So I'm wondering why > you're seeing such an optimization by repeating those calls. > > Perhaps the rcu_user_* somehow aren't actually called from > __context_tracking_enter()...? Some bug in context tracking? > Otherwise it's a curious side effect. David is working with v4.15. Is this maybe something that has changed since then? Thanx, Paul