linux-kernel - Re: [PATCH RT v2 2/3] sched: migrate_enable: Use sleeping

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Date:   Mon, 26 Aug 2019 11:12:47 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Scott Wood <swood@...hat.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Joel Fernandes <joel@...lfernandes.org>,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Clark Williams <williams@...hat.com>
Subject: Re: [PATCH RT v2 2/3] sched: migrate_enable: Use sleeping_lock to
 indicate involuntary sleep

On Mon, Aug 26, 2019 at 12:49:22PM -0500, Scott Wood wrote:
> 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.

Understood!  And yes, that was your answer to my question.  Me, I was
just answering Sebastian's question.  ;-)

							Thanx, Paul