Re: [PATCH RT v2 2/3] sched: migrate_enable: Use sleeping_lock to indicate involuntary sleep

From: Joel Fernandes
Date: Tue Aug 27 2019 - 09:08:58 EST

On Tue, Aug 27, 2019 at 11:23:33AM +0200, Sebastian Andrzej Siewior wrote:
> > However, if this was instead an rcu_read_lock() critical section within
> > a PREEMPT=y kernel, then if a schedule() occured within stop_one_task(),
> > RCU would consider that critical section to be preempted. This means
> > that any RCU grace period that is blocked by this RCU read-side critical
> > section would remain blocked until stop_one_cpu() resumed, returned,
> > and so on until the matching rcu_read_unlock() was reached. In other
> > words, RCU would consider that RCU read-side critical section to span
> > the call to stop_one_cpu() even if stop_one_cpu() invoked schedule().
> Isn't that my example from above and what we do in RT? My understanding
> is that this is the reason why we need BOOST on RT otherwise the RCU
> critical section could remain blocked for some time.

Not just for boost, it is needed to block the grace period itself on
PREEMPT=y. On PREEMPT=y, if rcu_note_context_switch() happens in middle of a
rcu_read_lock() reader section, then the task is added to a blocked list
(rcu_preempt_ctxt_queue). Then just after that, the CPU reports a QS state
(rcu_qs()) as you can see in the PREEMPT=y implementation of
rcu_note_context_switch(). Even though the CPU has reported a QS, the grace
period will not end because the preempted (or block as could be in -rt) task
is still blocking the grace period. This is fundamental to the function of
Preemptible-RCU where there is the concept of tasks blocking a grace period,
not just CPUs.

I think what Paul is trying to explain AIUI (Paul please let me know if I
missed something):

(1) Anyone calling rcu_note_context_switch() and expecting it to respect
RCU-readers that are readers as a result of interrupt disabled regions, have
incorrect expectations. So calling rcu_note_context_switch() has to be done

(2) Disabling interrupts is "generally" implied as an RCU-sched flavor
reader. However, invoking rcu_note_context_switch() from a disabled interrupt
region is *required* for rcu_note_context_switch() to work correctly.

(3) On PREEMPT=y kernels, invoking rcu_note_context_switch() from an
interrupt disabled region does not mean that that the task will be added to a
blocked list (unless it is also in an RCU-preempt reader) so
rcu_note_context_switch() may immediately report a quiescent state and
nothing blockings the grace period.
So callers of rcu_note_context_switch() must be aware of this behavior.

(4) On PREEMPT=n, unlike PREEMPT=y, there is no blocked list handling and so
nothing will block the grace period once rcu_note_context_switch() is called.
So any path calling rcu_note_context_switch() on a PREEMPT=n kernel, in the
middle of something that is expected to be an RCU reader would be really bad
from an RCU view point.

Probably, we should add this all to documentation somewhere.


- Joel

> > On the other hand, within a PREEMPT=n kernel, the call to schedule()
> > would split even an rcu_read_lock() critical section. Which is why I
> > asked earlier if sleeping_lock_inc() and sleeping_lock_dec() are no-ops
> > in !PREEMPT_RT_BASE kernels. We would after all want the usual lockdep
> > complaints in that case.
> sleeping_lock_inc() +dec() is only RT specific. It is part of RT's
> spin_lock() implementation and used by RCU (rcu_note_context_switch())
> to not complain if invoked within a critical section.
> > Does that help, or am I missing the point?
> >
> > Thanx, Paul
> Sebastian