From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756133Ab2BAKof (ORCPT ); Wed, 1 Feb 2012 05:44:35 -0500 Received: from mx1.redhat.com ([209.132.183.28]:54266 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753954Ab2BAKod (ORCPT ); Wed, 1 Feb 2012 05:44:33 -0500 Message-ID: <4F29178A.1090306@redhat.com> Date: Wed, 01 Feb 2012 12:44:26 +0200 From: Avi Kivity User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:9.0) Gecko/20111222 Thunderbird/9.0 MIME-Version: 1.0 To: Peter Zijlstra CC: paulmck@linux.vnet.ibm.com, Oleg Nesterov , linux-kernel , Marcelo Tosatti , KVM list Subject: Re: [RFC][PATCH] srcu: Implement call_srcu() References: <1328016724.2446.229.camel@twins> <4F27F0E6.1040309@redhat.com> <1328017807.2446.230.camel@twins> <20120131222447.GH2391@linux.vnet.ibm.com> <1328091749.2760.34.camel@laptop> In-Reply-To: <1328091749.2760.34.camel@laptop> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/01/2012 12:22 PM, Peter Zijlstra wrote: > One of the things I was thinking of is adding a sequence counter in the > per-cpu data. Using that we could do something like: > > unsigned int seq1 = 0, seq2 = 0, count = 0; > int cpu, idx; > > idx = ACCESS_ONCE(sp->completions) & 1; > > for_each_possible_cpu(cpu) > seq1 += per_cpu(sp->per_cpu_ref, cpu)->seq; > > for_each_possible_cpu(cpu) > count += per_cpu(sp->per_cpu_ref, cpu)->c[idx]; > > for_each_possible_cpu(cpu) > seq2 += per_cpu(sp->per_cpu_ref, cpu)->seq; > > /* > * there's no active references and no activity, we pass > */ > if (seq1 == seq2 && count == 0) > return; > > synchronize_srcu_slow(); > > > This would add a fast-path which should catch the case Avi outlined > where we call sync_srcu() when there's no other SRCU activity. Sorry, I was inaccurate. In two of the cases indeed we don't expect guest activity, and we're okay with waiting a bit if there is guest activity - when we're altering the guest physical memory map. But the third case does have concurrent guest activity with synchronize_srcu_expedited() and we still need it fast - that's when userspace reads the dirty bitmap log of a running guest and replaces it with a new bitmap. There may be a way to convert it to call_srcu() though. Without synchronize_srcu_expedited(), kvm sees both the old and the new bitmaps, but that's fine, since the dirty bits will go *somewhere*, and we can pick them up later in call_srcu(). The only problem is if this is the very last call to kvm_vm_ioctl_get_dirty_log(), and the callback triggers after it returns - we end up with a bag of bits with not one to return them to. Maybe we can detect this conditions (all vcpus ought to be stopped), and do something like: if (all vcpus stopped) { /* no activity, this should be fast */ synchronize_srcu() /* collect and return bits */ } else { call_srcu(collect bits) } still a snag - we can't reliably detect that all vcpus are stopped, they may be just resting in userspace, and restart while synchronize_srcu() is running. Marcelo? -- error compiling committee.c: too many arguments to function