From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini 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 14:35:38 +0200 Message-ID: <55F9621A.7070803@redhat.com> References: <55F8097A.7000206@de.ibm.com> <20150915130550.GC16853@twins.programming.kicks-ass.net> <55F81EE2.4090708@de.ibm.com> <55F84A6B.1010207@redhat.com> <20150915173836.GO4029@linux.vnet.ibm.com> <55F92904.4090206@redhat.com> <55F92F04.1040706@de.ibm.com> <55F9326A.9070509@redhat.com> <20150916122249.GA28821@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 8bit Cc: Christian Borntraeger , paulmck@linux.vnet.ibm.com, Peter Zijlstra , Tejun Heo , Ingo Molnar , Linux Kernel Mailing List , KVM list To: Oleg Nesterov Return-path: In-Reply-To: <20150916122249.GA28821@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 16/09/2015 14:22, Oleg Nesterov wrote: > > The issue is that rcu_sync doesn't eliminate synchronize_sched, > > Yes, but it eliminates _expedited(). This is good, but otoh this means > that (say) individual __cgroup_procs_write() can take much more time. > However, it won't block the readers and/or disturb the whole system. According to Christian, removing the _expedited() "makes things worse" in that the system takes ages to boot up and systemd timeouts. So I'm still a bit wary about anything that uses RCU for the cgroups write side. However, rcu_sync is okay with him, so perhaps it is really really effective. Christian, can you instrument how many synchronize_sched (out of the 6479 cgroup_procs_write calls) are actually executed at boot with the rcu rework? Paolo > And percpu_up_write() doesn't do synchronize_sched() at all.