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: Tue, 15 Sep 2015 23:11:45 +0200 Message-ID: <55F88991.7040406@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> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Cc: Tejun Heo , Ingo Molnar , "linux-kernel@vger.kernel.org >> Linux Kernel Mailing List" , KVM list , Oleg Nesterov , Paul McKenney To: Paolo Bonzini , Peter Zijlstra Return-path: In-Reply-To: <55F84A6B.1010207@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org Am 15.09.2015 um 18:42 schrieb Paolo Bonzini: > > > On 15/09/2015 15:36, Christian Borntraeger wrote: >> I am wondering why the old code behaved in such fatal ways. Is there >> some interaction between waiting for a reschedule in the >> synchronize_sched writer and some fork code actually waiting for the >> read side to get the lock together with some rescheduling going on >> waiting for a lock that fork holds? lockdep does not give me an hints >> so I have no clue :-( > > It may just be consuming too much CPU usage. kernel/rcu/tree.c warns > about it: > > * if you are using synchronize_sched_expedited() in a loop, please > * restructure your code to batch your updates, and then use a single > * synchronize_sched() instead. > > and you may remember that in KVM we switched from RCU to SRCU exactly to > avoid userspace-controlled synchronize_rcu_expedited(). > > In fact, I would say that any userspace-controlled call to *_expedited() > is a bug waiting to happen and a bad idea---because userspace can, with > little effort, end up calling it in a loop. Right. This also implies that we should fix this for 4.2 - I guess. Christian