On Tue, Jul 07, 2015 at 06:30:30PM +0200, Oleg Nesterov wrote: > On 07/06, Frederic Weisbecker wrote: > > > > The call_usermodehelper_exec_[a]sync() kernel threads are created by > > khelper precisely because > > I think khelper should simply die. It doesn't make any sense today, > kmod.c should use some system wq instead. But see below. > > > Not only useless it even breaks nohz full. The housekeeping work > > (general kernel internal code that user doesn't care much about) is > > handled by a reduced set of CPUs in nohz full, precisely those that are > > not included by nohz_full= kernel parameters. For example unbound > > workqueues are handled by housekeeping CPUs. > > Confused... I do not see how workqueue_attrs->cpumask can depend on > tick_nohz_full_mask or housekeeping_mask. Could you explain?
People who want CPU isolation will likely write /sys/devices/virtual/workqueue/cpumask to a reduced set of CPUs, typically CPU 0 that is used for housekeeping in nohz full. In fact we should add the code which initialize wq_unbound_cpumask to housekeeping_mask automatically. So this cpumask is inherited to khelper because it is a singlethread workqueue. > > > @@ -223,9 +223,6 @@ static int call_usermodehelper_exec_async(void *data) > > flush_signal_handlers(current, 1); > > spin_unlock_irq(¤t->sighand->siglock); > > > > - /* We can run anywhere, unlike our parent keventd(). */ > > - set_cpus_allowed_ptr(current, cpu_all_mask); > > - > > I think this is fine, ->no_numa is true for khelper. > > But this means that after this change kmod.c can't use a system wq, > ->no_numa is false by default. And khelper is no_numa only because > it is __WQ_ORDERED, but kmod.c doesn't need really need__WQ_ORDERED, > except, again, this implies ->no_numa == T. I'm not sure what means no_numa in the context of workqueues, I guess it's about having system workqueues bound to one CPU or several in the same nodes. But indeed we can't use system workqueues because they are per-cpu and we inherit that. And it's ridiculous to call set_cpus_allowed_ptr() to fix that. Hence why we use a singlethread, even though we don't care about ordering. Ok I guess that's more or less what you just said :o) > So perhaps init_workqueues() should create another global > WQ_UNBOUND/no_numa workqueue_struct so that we could kill khelper_wq? > Or kmod.c can use system_unbound_wq, but then we need to keep this > set_cpus_allowed_ptr(). > > To me khelper_wq looks just annoying. That's a good idea. I can do that! Perhaps queuing there would be done through schedule_work_unbound() ? Or schedule_work_no_numa()? -- 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/
