On Tue, 7 Apr 2015, Jason Low wrote:
> On Tue, 2015-04-07 at 21:17 +0200, Thomas Gleixner wrote:
> > On Tue, 7 Apr 2015, Jason Low wrote:
> > > The lock shouldn't be used in get_next_timer_interrupt() either right?
> > >
> > > unsigned long get_next_timer_interrupt(unsigned long now)
> > > {
> > > ...
> > >
> > > #ifdef CONFIG_PREEMPT_RT_FULL
> > > /*
> > > * On PREEMPT_RT we cannot sleep here. If the trylock does not
> > > * succeed then we return the worst-case 'expires in 1 tick'
> > > * value. We use the rt functions here directly to avoid a
> > > * migrate_disable() call.
> > > */
> > > if (!spin_do_trylock(&base->lock))
> > > return now + 1;
> > > #else
> >
> > And how do you protect the walk of the timer wheel against a
> > concurrent insertion/removal?
>
> So I just wanted to mention that the issue also applies to
> get_next_timer_interrupt(), in addition to run_local_timers(), but if we
> really want to remove the lock there, can we always return "now + 1" for
> PREEMPT_RT_FULL?
There is a simpler solution for this: Disable NOHZ for RT. Because
that's what 'always return "now + 1"' does at runtime just with more
overhead.
We need a smarter solution for RT to avoid the lock completely.
Thanks,
tglx
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