On Mon, Feb 24, 2014 at 08:04:25PM +0100, Daniel Lezcano wrote: > >+ /* > >+ * Since we fell out of the loop above, we know > >+ * TIF_NEED_RESCHED must be set, propagate it into > >+ * PREEMPT_NEED_RESCHED. > >+ * > >+ * This is required because for polling idle loops we will > >+ * not have had an IPI to fold the state for us. > >+ */ > >+ preempt_set_need_resched(); > > tick_nohz_idle_exit(); > > schedule_preempt_disabled(); > > So IIUC, the mainloop has two states: one where it is blocked on a HLT/WFI > instruction (or about to enter/ exit this state) and another one outside of > this blocking section.
Almost; on x86 we have an monitor/mwait construct that blocks waiting for a cacheline write. We point it at the thread_info->flags line. So the TIF_NEED_RESCHED write from the other CPU wakes us up. So no need to send an IPI after we write that flag. > When the idle task is blocked on HLT/WFI, it needs the IPI-reschedule in > order to be woken up and rescheduled. But if it is outside this section, the > idle task is not waiting for an interrupt and an expensive IPI can be saved > by just setting the TS_POLLING flag, the scheduler will check this flag and > won't send the IPI. > > But 'set_preempt_need_resched' is called from the IPI handler. So if no IPI > is sent because the idle task is in polling state, we have to set it > ourself. > > Now, the difference between the old code with 'tif_need_resched()' is > because we don't need to check it because it is always true. > > Am I right ? Yah, also it closes a very narrow window where TIF_NEED_RESCHED wasn't set at the end of the while (!need_resched()) but is at the top. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
