On Wed, Feb 12, 2014 at 04:59:52PM +0100, Frederic Weisbecker wrote: > 2014-02-12 11:13 GMT+01:00 Peter Zijlstra <pet...@infradead.org>: > > On Tue, Feb 11, 2014 at 02:34:11PM -0800, Andy Lutomirski wrote: > >> On Tue, Feb 11, 2014 at 1:21 PM, Thomas Gleixner <t...@linutronix.de> > >> wrote: > >> >> A small number of reschedule interrupts appear to be due to a race: > >> >> both resched_task and wake_up_idle_cpu do, essentially: > >> >> > >> >> set_tsk_need_resched(t); > >> >> smb_mb(); > >> >> if (!tsk_is_polling(t)) > >> >> smp_send_reschedule(cpu); > >> >> > >> >> The problem is that set_tsk_need_resched wakes the CPU and, if the CPU > >> >> is too quick (which isn't surprising if it was in C0 or C1), then it > >> >> could *clear* TS_POLLING before tsk_is_polling is read. > > > > Yeah we have the wrong default for the idle loops.. it should default to > > polling and only switch to !polling at the very last moment if it really > > needs an interrupt to wake. > > > > Changing this requires someone (probably me again :/) to audit all arch > > cpu idle drivers/functions. > > Looking at wake_up_idle_cpu(), we set need_resched and send the IPI. > On the other end, the CPU wakes up, exits the idle loop and even goes > to the scheduler while there is probably no task to schedule. > > I wonder if this is all necessary. All we need is the timer to be > handled by the dynticks code to re-evaluate the next tick. So calling > irq_exit() -> tick_nohz_irq_exit() from the scheduler_ipi() should be > enough.
No no, the idea was to NOT send IPIs. So falling out of idle by writing TIF_NEED_RESCHED and having the idle loop fixup the timers on its way back to idle is what you want. -- 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/
