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 13:50:28 +0200 Message-ID: <55F95784.8070608@de.ibm.com> References: <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> <55F94B38.5060308@de.ibm.com> <20150916110351.GA17047@htj.duckdns.org> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Cc: "Paul E. McKenney" , Paolo Bonzini , Peter Zijlstra , Ingo Molnar , "linux-kernel@vger.kernel.org >> Linux Kernel Mailing List" , KVM list , Oleg Nesterov To: Tejun Heo Return-path: In-Reply-To: <20150916110351.GA17047@htj.duckdns.org> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org Am 16.09.2015 um 13:03 schrieb Tejun Heo: > Hello, > > On Wed, Sep 16, 2015 at 12:58:00PM +0200, Christian Borntraeger wrote: >> 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. > > Booting would usually be the hottest operation for that and it's still > *relatively* cold path compared to the reader side which is task > fork/exit paths. The whole point is shift overhead from hotter reader > side. Can you see problems with percpu_rwsem rework? As I said, it seems the rcu tree with that change seems to work fine on my system. This needs a more testing on other machines, though. I guess a revert plus a re-add in the 4.4 merge window should give us enough test coverage. Christian