On Wed, Aug 29, 2007 at 02:52:04PM +0400, Oleg Nesterov wrote: > 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.
Well, the comment for cpuset_cpus_allowed() says /* * Description: Returns the cpumask_t cpus_allowed of the cpuset * attached to the specified @tsk. Guaranteed to return some non-empty * subset of cpu_online_map, even if this means going outside the * tasks cpuset. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ **/^^^^^^^^^^^^^ Since this behaviour has been documented, I presume there is a reason behind it. So either we're incorrectly using cpuset_cpus_allowed in sched_setaffinity or we're missing something subtle :) Thanks and Regards gautham. -- Gautham R Shenoy Linux Technology Center IBM India. "Freedom comes with a price tag of responsibility, which is still a bargain, because Freedom is priceless!" - 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/