On Wed, Nov 30, 2016 at 01:13:03PM -0600, Josh Poimboeuf wrote:
> On Tue, Nov 29, 2016 at 12:32:59PM -0800, Paul E. McKenney wrote:
> > On Tue, Nov 29, 2016 at 12:07:11PM -0800, Paul E. McKenney wrote:
> > > On Tue, Nov 29, 2016 at 08:52:04PM +0100, Peter Zijlstra wrote:
> > > > On Tue, Nov 29, 2016 at 11:39:35AM -0800, Paul E. McKenney wrote:
> > > > > On Tue, Nov 29, 2016 at 06:10:38PM +0100, Peter Zijlstra wrote:
> > > >
> > > > > > It mostly works, most of the time, and that seems to be what Linus
> > > > > > wants, since its really the best we can have given the constraints.
> > > > > > But
> > > > > > for debugging, when you have a UART, it totally blows.
> > > > >
> > > > > UART??? They still make those things??? ;-)
> > > >
> > > > Yes, most computer like devices actually have them, trouble is, most
> > > > consumer devices don't have the pins exposed. Luckily most server class
> > > > hardware still does.
> > > >
> > > > And they're absolutely _awesome_ for debugging; getting data out is a
> > > > matter of trivial MMIO poll loops. Rock solid stuff.
> > >
> > > They very clearly need to bring the baud rate into the current millenium,
> > > many tens of Mbaud at the -very- least.
> >
> > On a more practical note...
> >
> > Currently, cond_resched_rcu_qs() is not permitted to be invoked until
> > after the scheduler has started. However, it appears that there is some
> > kernel code that can loop for quite some time at runtime, but which also
> > executes during early boot. So it would be good to make it so that
> > cond_resched_rcu_qs() could be called at boot.
> >
> > One approach would be to check rcu_scheduler_active, but this isn't
> > defined in normal Tiny RCU builds. I can expand Tiny RCU, or I can
> > kludge the non-CONFIG_DEBUG_LOCK_ALLOC value of rcu_scheduler_active
> > to false (with this latter being the current state). But it occurred
> > to me that I could also condition on !is_idle_task(), given that idle
> > tasks shouldn't ever be invoking the scheduler anyway.
>
> This question was probably intended for other folks, but I should point
> out that idle tasks *do* invoke the scheduler. cpu_idle_loop() calls
> schedule_preempt_disabled().
Good point. My next fallback is that idle loops should not be running
for long periods of time within RCU_NONIDLE(). Does that work?
Thanx, Paul
> > So is the following a sensible approach, or should I look elsewhere?
> >
> > #define cond_resched_rcu_qs() \
> > do { \
> > if (!is_idle_task(current) && !cond_resched()) \
> > rcu_note_voluntary_context_switch(current); \
> > } while (0)
> >
> > Thanx, Paul
> >
>
> --
> Josh
>
