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

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Message-ID: <20190824031014.GB2731@google.com>
Date:   Fri, 23 Aug 2019 23:10:14 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Scott Wood <swood@...hat.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Paul E . McKenney" <paulmck@...ux.ibm.com>,
        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 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:
> > On 2019-08-21 18:19:05 [-0500], Scott Wood wrote:
> > > Without this, rcu_note_context_switch() will complain if an RCU read
> > > lock is held when migrate_enable() calls stop_one_cpu().
> > > 
> > > Signed-off-by: Scott Wood <swood@...hat.com>
> > > ---
> > > v2: Added comment.
> > > 
> > > If my migrate disable changes aren't taken, then pin_current_cpu()
> > > will also need to use sleeping_lock_inc() because calling
> > > __read_rt_lock() bypasses the usual place it's done.
> > > 
> > >  include/linux/sched.h    | 4 ++--
> > >  kernel/rcu/tree_plugin.h | 2 +-
> > >  kernel/sched/core.c      | 8 ++++++++
> > >  3 files changed, 11 insertions(+), 3 deletions(-)
> > > 
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -7405,7 +7405,15 @@ void migrate_enable(void)
> > >  			unpin_current_cpu();
> > >  			preempt_lazy_enable();
> > >  			preempt_enable();
> > > +
> > > +			/*
> > > +			 * sleeping_lock_inc suppresses a debug check for
> > > +			 * sleeping inside an RCU read side critical section
> > > +			 */
> > > +			sleeping_lock_inc();
> > >  			stop_one_cpu(task_cpu(p), migration_cpu_stop, &arg);
> > > +			sleeping_lock_dec();
> > 
> > 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.

thanks,

 - Joel