* Peter Zijlstra ([EMAIL PROTECTED]) wrote: > On Sat, 2007-07-14 at 14:42 -0400, Mathieu Desnoyers wrote: > > > Note: (or we could say FIXME) > > Is we ever want to check migration pending in assembly code, we will have to > > make sure we test the right thread flag bits on each architectures. Care > > should > > also be taken to check that the thread flags used won't trigger false > > positives > > in non selective asm thread flag checks. > > > > FIXME (HOTPLUG) : > > > > > > /* Affinity changed (again). */ > > > > if (!cpu_isset(dest_cpu, p->cpus_allowed)) > > > > goto out; > > > > > > > > on_rq = p->se.on_rq; > > > > +#ifdef CONFIG_PREEMPT > > > > + if (!on_rq && task_thread_info(p)->migrate_count) > > > > + goto out; > > > > +#endif > > > > > > This means that move_task_off_dead_cpu() will spin until the task will be > > > scheduled > > > on the dead CPU. Given that we hold tasklist_lock and irqs are disabled, > > > this > > > may > > > never happen. > > > > > > > Yes. My idea to fix this issue is the following: > > > > If a thread has non zero migrate_count, we should still move it to a > > different CPU upon hotplug cpu removal, even if this thread resists > > migration. Care should be taken to send _all_ such threads to the _same_ > > CPU so they don't race for the per-cpu ressources. Does it make sense ? > > > > We would have to keep the CPU affinity of the threads running on the > > wrong CPU until they end their migrate disabled section, so that we can > > put them back on their original CPU if it goes back online, otherwise we > > could end up with concurrent per-cpu variables accesses. > > > > (I'll wait for reply before coding a solution for this CPU HOTPLUG > > related problem) > > What would, aside from technical issues, be the problem with making > migration_disable() delay CPU_DOWN until migration_enable()? >
Because if we thing a little further, migration disabling could be a very interesting way to provide cheap per-cpu data structure access to user-space. But we would not want user-space processes to hold CPU_DOWN forever... -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - 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/
