From mboxrd@z Thu Jan 1 00:00:00 1970 From: Quentin Perret Subject: Re: [PATCH v4 1/5] sched/topology: Add check to backup comment about hotplug lock Date: Fri, 15 Jun 2018 09:39:51 +0100 Message-ID: <20180615083951.GO17720@e108498-lin.cambridge.arm.com> References: <20180613121711.5018-1-juri.lelli@redhat.com> <20180613121711.5018-2-juri.lelli@redhat.com> <20180614093324.7ea45448@gandalf.local.home> <20180614134234.GC12032@localhost.localdomain> <20180614094747.390357ec@gandalf.local.home> <20180614135040.GE12032@localhost.localdomain> <20180614135800.GM17720@e108498-lin.cambridge.arm.com> <20180614141118.GG12032@localhost.localdomain> <20180614141818.GN17720@e108498-lin.cambridge.arm.com> <20180614143037.GH12032@localhost.localdomain> Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: <20180614143037.GH12032@localhost.localdomain> Sender: linux-kernel-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Juri Lelli Cc: Steven Rostedt , peterz@infradead.org, mingo@redhat.com, linux-kernel@vger.kernel.org, luca.abeni@santannapisa.it, claudio@evidence.eu.com, tommaso.cucinotta@santannapisa.it, bristot@redhat.com, mathieu.poirier@linaro.org, lizefan@huawei.com, cgroups@vger.kernel.org On Thursday 14 Jun 2018 at 16:30:37 (+0200), Juri Lelli wrote: > On 14/06/18 15:18, Quentin Perret wrote: > > On Thursday 14 Jun 2018 at 16:11:18 (+0200), Juri Lelli wrote: > > > On 14/06/18 14:58, Quentin Perret wrote: > > > > > > [...] > > > > > > > Hmm not sure if this can help but I think that rebuild_sched_domains() > > > > does _not_ take the hotplug lock before calling partition_sched_domains() > > > > when CONFIG_CPUSETS=n. But it does take it for CONFIG_CPUSETS=y. > > > > > > Did you mean cpuset_mutex? > > > > Nope, I really meant the cpu_hotplug_lock ! > > > > With CONFIG_CPUSETS=n, rebuild_sched_domains() calls > > partition_sched_domains() directly: > > > > https://elixir.bootlin.com/linux/latest/source/include/linux/cpuset.h#L255 > > > > But with CONFIG_CPUSETS=y, rebuild_sched_domains() calls, > > rebuild_sched_domains_locked(), which calls get_online_cpus() which > > calls cpus_read_lock(), which does percpu_down_read(&cpu_hotplug_lock). > > And all that happens before calling partition_sched_domains(). > > Ah, right! > > > So yeah, the point I was trying to make is that there is an inconsistency > > here, maybe for a good reason ? Maybe related to the issue you're seeing ? > > The config that came with the 0day splat was indeed CONFIG_CPUSETS=n. > > So, in this case IIUC we hit the !doms_new branch of partition_sched_ > domains, which uses cpu_active_mask (and cpu_possible_mask indirectly). > Should this be still protected by the hotplug lock then? Hmm I'm not sure ... But looking at your call trace, it seems that the issue happens when sched_cpu_deactivate() is called (not sure why this is called during boot BTW ?), which calls cpuset_update_active_cpus(). And again, for CONFIG_CPUSETS=n, that defaults to a raw call to partition_sched_domain(), but with ndoms_new=1, and no lock taken. I'm still not sure if this is done like that for a good reason, or if this is actually an issue that this patch caught nicely ... Quentin