On Tue, Dec 17, 2013 at 03:27:14PM -0800, Paul E. McKenney wrote: > On Tue, Dec 17, 2013 at 11:51:22PM +0100, Frederic Weisbecker wrote: > > The purpose of the full system idle detection is to notify the CPU > > handling the timekeeping when the rest of the system is idle so that it > > can sleep when nobody needs the jiffies nor GTOD to be maintained. > > > > Now this machinery excludes CPU 0 itself from the range of the idle > > detection because if CPU 0 has any non-idle task to execute, it is going > > to restart its own tick since it's guaranteed to be outside the full > > dynticks range. And as it is the only eligible timekeeper when > > CONFIG_NO_HZ_FULL=y anyway, it can handle the timekeeping duty for > > and by itself. > > > > Still we also plan to extend the timekeepers affinity and allow every CPU > > outside the full dynticks range to handle the timekeeping duty, not just > > CPU 0. > > > > So once we reach that step, we can state that all CPUs that are not > > full dynticks can be excluded from the full system idle detection, > > simply because those CPUs share the same property than CPU 0 today. When > > a non full dynticks CPU needs to run some busy task, it restarts its > > tick and handles the timekeeping duty for its own needs as is currently > > done under CONFIG_NO_HZ_IDLE=y. > > > > To prepare for this support in the sysidle detection, we can use the > > tick_timekeeping_cpu() API which checks if a CPU is allowed to handle > > timekeeping duty. If so we can conclude that it's not full dynticks and > > it can maintain timekeeping by itself and as such it can be excluded > > from the sysidle detection. > > > > Signed-off-by: Frederic Weisbecker <fweis...@gmail.com> > > Cc: Thomas Gleixner <t...@linutronix.de> > > Cc: Ingo Molnar <mi...@kernel.org> > > Cc: Peter Zijlstra <pet...@infradead.org> > > Cc: Steven Rostedt <rost...@goodmis.org> > > Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com> > > Cc: John Stultz <john.stu...@linaro.org> > > Cc: Alex Shi <alex....@linaro.org> > > Cc: Kevin Hilman <khil...@linaro.org> > > Reviewed-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
Thanks! > > A few comments below as well. > > > --- > > kernel/rcu/tree_plugin.h | 8 ++++---- > > 1 file changed, 4 insertions(+), 4 deletions(-) > > > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > > index 6abb03d..08004da 100644 > > --- a/kernel/rcu/tree_plugin.h > > +++ b/kernel/rcu/tree_plugin.h > > The rcu_sysidle_force_exit() function uses tick_do_timer_cpu, but > presumably needs to continue doing so in order to whack the right > CPU over the head. I am happy to defer worrying about the interaction > with multiple timekeeping CPUs for the moment. ;-) So yeah, I changed that in "nohz: Wake up timekeeper on exit from sysidle state". We always target CPU 0 for the IPI. That's to start simple as CPU 0 can't be offlined. But we certainly want to optimize that by kicking a potential timekeeper that is not idle. But that require a lookup like get_timer_nohz_target() and some safety against CPU hotplug. So that needs more thought :) > > > @@ -2539,7 +2539,7 @@ static void rcu_sysidle_exit(struct rcu_dynticks > > *rdtp, int irq) > > * invoke rcu_sysidle_force_exit() directly if it does anything > > * more than take a scheduling-clock interrupt. > > */ > > - if (smp_processor_id() == tick_do_timer_cpu) > > + if (tick_timekeeping_cpu(smp_processor_id())) > > return; > > > > /* Update system-idle state: We are clearly no longer fully idle! */ > > @@ -2563,10 +2563,10 @@ static void rcu_sysidle_check_cpu(struct rcu_data > > *rdp, bool *isidle, > > * is an offline or the timekeeping CPU, nothing to do. > > */ > > if (!*isidle || rdp->rsp != rcu_sysidle_state || > > - cpu_is_offline(rdp->cpu) || rdp->cpu == tick_do_timer_cpu) > > + cpu_is_offline(rdp->cpu) || tick_timekeeping_cpu(rdp->cpu)) > > return; > > if (rcu_gp_in_progress(rdp->rsp)) > > - WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu); > > + WARN_ON_ONCE(!tick_timekeeping_cpu(smp_processor_id())); > > > > /* Pick up current idle and NMI-nesting counter and check. */ > > cur = atomic_read(&rdtp->dynticks_idle); > > The rcu_bind_gp_kthread() uses tick_do_timer_cpu to figure out where > to run. Is there some CPU mask that it should use instead once there > can be multiple timekeeping CPUs? Good point, we'll need to build one, or use ~nohz_full_mask Thanks. > > > @@ -2729,7 +2729,7 @@ bool rcu_sys_is_idle(void) > > static struct rcu_sysidle_head rsh; > > int rss = ACCESS_ONCE(full_sysidle_state); > > > > - if (WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu)) > > + if (WARN_ON_ONCE(!tick_timekeeping_cpu(smp_processor_id()))) > > return false; > > > > /* Handle small-system case by doing a full scan of CPUs. */ > > -- > > 1.8.3.1 > > > -- 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/
