From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christian Borntraeger Subject: Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace signal_struct->group_rwsem with a global percpu_rwsem) causes regression for libvirt/kvm Date: Wed, 16 Sep 2015 12:58:00 +0200 Message-ID: <55F94B38.5060308@de.ibm.com> References: <55F8097A.7000206@de.ibm.com> <20150915130550.GC16853@twins.programming.kicks-ass.net> <55F81EE2.4090708@de.ibm.com> <55F84A6B.1010207@redhat.com> <55F88991.7040406@de.ibm.com> <20150915212622.GC495@htj.duckdns.org> <20150915213830.GR4029@linux.vnet.ibm.com> <20150915222811.GD495@htj.duckdns.org> <20150915233818.GU4029@linux.vnet.ibm.com> <20150916012415.GC25658@htj.duckdns.org> <55F91DDB.6070207@de.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Cc: Paolo Bonzini , Peter Zijlstra , Ingo Molnar , "linux-kernel@vger.kernel.org >> Linux Kernel Mailing List" , KVM list , Oleg Nesterov To: Tejun Heo , "Paul E. McKenney" Return-path: In-Reply-To: <55F91DDB.6070207@de.ibm.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org Am 16.09.2015 um 09:44 schrieb Christian Borntraeger: > Am 16.09.2015 um 03:24 schrieb Tejun Heo: >> Hello, Paul. >> >> On Tue, Sep 15, 2015 at 04:38:18PM -0700, Paul E. McKenney wrote: >>> Well, the decision as to what is too big for -stable is owned by the >>> -stable maintainers, not by me. >> >> Is it tho? Usually the subsystem maintainer knows the best and has >> most say in it. I was mostly curious whether you'd think that the >> changes would be too risky. If not, great. >> >>> I am suggesting trying the options and seeing what works best, then >>> working to convince people as needed. >> >> Yeah, sure thing. Let's wait for Christian. > > Well, I have optimized my testcase now that is puts enough pressure to > the system to confuses system (the older 209 version, which still has > some event loop issues) that systemd restarts the journal deamon and does > several other recoveries. > To avoid regressions - even for somewhat shaky userspaces - we should > consider a revert for 4.2 stable. > There are several followup patches, which makes the revert non-trivial, > though. > > The rework of the percpu rwsem seems to work fine, but we are beyond the > merge window so 4.4 seems better to me. (and consider a revert for 4.3) FWIW, I added a printk to percpu_down_write. With KVM and uprobes disabled, just booting up a fedora20 gives me __6749__ percpu_down_write calls on 4.2. systemd seems to do that for the processes. So a revert is really the right thing to do. In fact, I dont know if the rcu_sync_enter rework is enough. With systemd setting the cgroup seem to be NOT a cold/seldom case. Christian