On Mon, Aug 29, 2016 at 12:40:32PM -0400, Chris Metcalf wrote: > On 8/29/2016 12:33 PM, Peter Zijlstra wrote: > >On Tue, Aug 16, 2016 at 05:19:27PM -0400, Chris Metcalf wrote: > >>+ /* > >>+ * Request rescheduling unless we are in full dynticks mode. > >>+ * We would eventually get pre-empted without this, and if > >>+ * there's another task waiting, it would run; but by > >>+ * explicitly requesting the reschedule, we may reduce the > >>+ * latency. We could directly call schedule() here as well, > >>+ * but since our caller is the standard place where schedule() > >>+ * is called, we defer to the caller. > >>+ * > >>+ * A more substantive approach here would be to use a struct > >>+ * completion here explicitly, and complete it when we shut > >>+ * down dynticks, but since we presumably have nothing better > >>+ * to do on this core anyway, just spinning seems plausible. > >>+ */ > >>+ if (!tick_nohz_tick_stopped()) > >>+ set_tsk_need_resched(current); > >This is broken.. and it would be really good if you don't actually need > >to do this. > > Can you elaborate? We clearly do want to wait until we are in full > dynticks mode before we return to userspace. > > We could do it just in the prctl() syscall only, but then we lose the > ability to implement the NOSIG mode, which can be a convenience. So this isn't spelled out anywhere. Why does this need to be in the return to user path? > Even without that consideration, we really can't be sure we stay in > dynticks mode if we disable the dynamic tick, but then enable interrupts, > and end up taking an interrupt on the way back to userspace, and > it turns the tick back on. That's why we do it here, where we know > interrupts will stay disabled until we get to userspace. But but but.. task_isolation_enter() is explicitly ran with IRQs _enabled_!! It even WARNs if they're disabled. > So if we are doing it here, what else can/should we do? There really > shouldn't be any other tasks waiting to run at this point, so there's > not a heck of a lot else to do on this core. We could just spin and > check need_resched and signal status manually instead, but that > seems kind of duplicative of code already done in our caller here. What !? I really don't get this, what are you waiting for? Why is rescheduling making things better. -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
