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

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Message-ID: <20190827160619.GA55873@google.com>
Date:   Tue, 27 Aug 2019 12:06:19 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Scott Wood <swood@...hat.com>, 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 Tue, Aug 27, 2019 at 08:58:13AM -0700, Paul E. McKenney wrote:
> On Tue, Aug 27, 2019 at 09:08:53AM -0400, Joel Fernandes wrote:
> > On Tue, Aug 27, 2019 at 11:23:33AM +0200, Sebastian Andrzej Siewior wrote:
> > [snip]
> > > > 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
> > carefully.
> > 
> > (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.
> 
> I think that Sebastian understands this and was using the example of RCU
> priority boosting to confirm his understanding.  But documentation would
> be good.  Extremely difficult to keep current, but good.  I believe that
> the requirements document does cover this.

Oh ok, got it. Sorry about the noise then!

(In a way, I was just thinking out loud since this is a slightly confusing
 topic :-P and an archive link to this discussion serves a great purpose in
 my notes :-D :-)).

thanks!

 - Joel