Re: [PATCH 2/4] timer: relax tick stop in idle entry

From: Jacob Pan
Date: Thu Nov 19 2015 - 12:44:46 EST

On Mon, 16 Nov 2015 16:09:10 -0800
Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx> wrote:

> > > For the second case, which is much more rare, I think we do have
> > > next timer exactly one tick away. Just don't know why tick will
> > > continue into idle loop.
> >
> > Well, it should not be hard to figure that out. There are not so
> > many checks involved when tick_nohz_irq_exit() is called.
> Thanks for the tip, I found the cause is in
> int idle_cpu(int cpu)
> {
> if (rq->nr_running)
> return 0;
> Since we only take into account of cfs_rq runnable taking over
> cfs_rq->nr_running when forced_idle is set.
I am not sure what is the best solution. It seems I can add additional
checks like this.
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3520,9 +3520,14 @@ int idle_cpu(int cpu)
if (rq->curr != rq->idle)
return 0;

- if (rq->nr_running)
- return 0;
+ if (rq->nr_running) {
+ /* if cfs_rq is in forced idle, nr_running could be
nonzero but still in idle */
+ if ((rq->nr_running != rq->cfs.h_nr_running) ||
+ cfs_rq_runnable(&rq->cfs))
+ return 0;
+ }

To recap the problem statement.
1. When entering idle loop tick_nohz_stop_sched_tick() checks if the
next timer interrupt is exactly one tick away. if so, it will not stop
it to avoid threshing timer disable and enable.
2. so it relies on the next round tick_nohz_irq_exit() to have another
chance to stop the tick
3. with idle injection rq->nr_running could be nonzero when in idle
4. tick_nohz_irq_exit() will not be called if !idle_cpu()

However, idle_cpu() is used by many other callers, e.g. load balance.
Do we want to consider forced idle in those cases? Or we can forgo this
case and ignore it?

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