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

[Scott Wood <swood@redhat.com>]

On Mon, 2019-08-26 at 09:29 -0700, Paul E. McKenney wrote:
> On Mon, Aug 26, 2019 at 05:25:23PM +0200, Sebastian Andrzej Siewior wrote:
> > On 2019-08-23 23:10:14 [-0400], Joel Fernandes wrote:
> > > On Fri, Aug 23, 2019 at 02:28:46PM -0500, Scott Wood wrote:
> > > > On Fri, 2019-08-23 at 18:20 +0200, Sebastian Andrzej Siewior wrote:
> > > > > this looks like an ugly hack. This sleeping_lock_inc() is used
> > > > > where we
> > > > > actually hold a sleeping lock and schedule() which is okay. But
> > > > > this
> > > > > would mean we hold a RCU lock and schedule() anyway. Is that okay?
> > > > 
> > > > Perhaps the name should be changed, but the concept is the same --
> > > > RT-
> > > > specific sleeping which should be considered involuntary for the
> > > > purpose of
> > > > debug checks.  Voluntary sleeping is not allowed in an RCU critical
> > > > section
> > > > because it will break the critical section on certain flavors of
> > > > RCU, but
> > > > that doesn't apply to the flavor used on RT.  Sleeping for a long
> > > > time in an
> > > > RCU critical section would also be a bad thing, but that also
> > > > doesn't apply
> > > > here.
> > > 
> > > I think the name should definitely be changed. At best, it is super
> > > confusing to
> > > call it "sleeping_lock" for this scenario. In fact here, you are not
> > > even
> > > blocking on a lock.
> > > 
> > > Maybe "sleeping_allowed" or some such.
> > 
> > The mechanism that is used here may change in future. I just wanted to
> > make sure that from RCU's side it is okay to schedule here.
> 
> Good point.
> 
> The effect from RCU's viewpoint will be to split any non-rcu_read_lock()
> RCU read-side critical section at this point.  This alrady happens in a
> few places, for example, rcu_note_context_switch() constitutes an RCU
> quiescent state despite being invoked with interrupts disabled (as is
> required!).  The __schedule() function just needs to understand (and does
> understand) that the RCU read-side critical section that would otherwise
> span that call to rcu_node_context_switch() is split in two by that call.
> 
> 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().
> 
> 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.

migrate_enable() is PREEMPT_RT_BASE-specific -- this code won't execute at
all with PREEMPT=n.

-Scott