From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753563AbbIPJMU (ORCPT ); Wed, 16 Sep 2015 05:12:20 -0400 Received: from mx1.redhat.com ([209.132.183.28]:57195 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752437AbbIPJMP (ORCPT ); Wed, 16 Sep 2015 05:12:15 -0400 Subject: Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace signal_struct->group_rwsem with a global percpu_rwsem) causes regression for libvirt/kvm To: Christian Borntraeger , paulmck@linux.vnet.ibm.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> Cc: Peter Zijlstra , Tejun Heo , Ingo Molnar , "linux-kernel@vger.kernel.org >> Linux Kernel Mailing List" , KVM list , Oleg Nesterov From: Paolo Bonzini X-Enigmail-Draft-Status: N1110 Message-ID: <55F9326A.9070509@redhat.com> Date: Wed, 16 Sep 2015 11:12:10 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 MIME-Version: 1.0 In-Reply-To: <55F92F04.1040706@de.ibm.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 16/09/2015 10:57, Christian Borntraeger wrote: > Am 16.09.2015 um 10:32 schrieb Paolo Bonzini: >> >> >> On 15/09/2015 19:38, Paul E. McKenney wrote: >>> Excellent points! >>> >>> Other options in such situations include the following: >>> >>> o Rework so that the code uses call_rcu*() instead of *_expedited(). >>> >>> o Maintain a per-task or per-CPU counter so that every so many >>> *_expedited() invocations instead uses the non-expedited >>> counterpart. (For example, synchronize_rcu instead of >>> synchronize_rcu_expedited().) >> >> Or just use ratelimit (untested): > > One of my tests was to always replace synchronize_sched_expedited with > synchronize_sched and things turned out to be even worse. Not sure if > it makes sense to test yopur in-the-middle approach? I don't think it applies here, since down_write/up_write is a synchronous API. If the revert isn't easy, I think backporting rcu_sync is the best bet. The issue is that rcu_sync doesn't eliminate synchronize_sched, it only makes it more rare. So it's possible that it isn't eliminating the root cause of the problem. Paolo