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 ? 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/
