On 08/29, Gautham R Shenoy wrote: > > On Tue, Aug 28, 2007 at 05:48:53PM +0400, Oleg Nesterov wrote: > > (cpu-hotplug experts cc'ed) > > > > On 08/25, Oleg Nesterov wrote: > > > > > > After the brief look at kernel/cpuset.c, it seems that attach_task() > > > should > > > guarantee that the task can't use CPUs outside of cpuset->cpus_allowed. > > > > > > But this looks racy wrt sched_setaffinity() which does > > > > > > cpus_allowed = cpuset_cpus_allowed(p); > > > // callback_mutex is free > > > set_cpus_allowed(p); > > > > > > What if attach_task()->set_cpus_allowed() happens in between? > > > > Actually, I think there is another problem, and cpuset_cpus_allowed() is > > just broken wrt CONFIG_HOTPLUG_CPU. > > > > Suppose that CONFIG_CPUSETS is true, but we don't use cpusets. In that > > case all tasks in system belong to the top_cpuset (btw, why cpuset_init() > > sets init_task.cpuset? this was already done by cpuset_init_early()), and > > we should have the same behaviour as without CONFIG_CPUSETS. > > > > By default, all tasks have ->cpus_allowed = CPU_MASK_ALL inherited from > > kernel_init(). This means that the task can use the new CPU right after > > cpu_up(). > > > > Now let's suppose that some task does sched_setaffinity(0, CPU_MASK_ALL). > > In that case, cpuset_cpus_allowed() sets ->cpus_allowed = cpu_online_map, > > and I think this is just wrong. Now that task doesn't see the new CPUs. > > > > Good point! > > A task's cpu_allowed mask can contain cpus which are offline. > And if those cpus exist in the intersection of the task's requested mask > and cpuset's cpu mask, why should we unset the offlined cpus from that > intersection? Either way the task is not going to run on the cpus while > they are in the offlined state. And on cpu_up, if the cpu is present in > the task's allowed mask, it can run on that cpu, which is a good thing. > > The two users of cpuset_cpus_allowed - sched_setaffinity and pdflush > don't seem to require the online cpu information. > > Paul, is there any particular reason why we need guarentee_online_cpus > to be called in cpuset_cpus_allowed ?
Note also that cpuset_cpus_allowed()->guarentee_online_cpus() easily allows the task to escape its ->cpuset, sched_setaffinity(cpumask_of_cpu(OFFLINE_CPU)) is enough. Another note, http://marc.info/?t=118823976000002 really needs something like guarentee_online_cpus(). However even in this case we shouldn't afaics "and" cpuset->cpus_allowed with cpu_online_map. Oleg. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
