Date:   Sat, 13 Jul 2019 07:18:50 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Byungchul Park <byungchul.park@....com>,
        linux-kernel@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        kernel-team@...roid.com
Subject: Re: [PATCH] rcuperf: Make rcuperf kernel test more robust for
 !expedited mode

On Fri, Jul 05, 2019 at 04:00:03PM -0400, Joel Fernandes wrote:
> On Fri, Jul 05, 2019 at 08:09:32AM -0700, Paul E. McKenney wrote:
> > On Fri, Jul 05, 2019 at 08:24:50AM -0400, Joel Fernandes wrote:
> > > On Fri, Jul 05, 2019 at 12:52:31PM +0900, Byungchul Park wrote:
> > > > On Thu, Jul 04, 2019 at 10:40:44AM -0700, Paul E. McKenney wrote:
> > > > > On Thu, Jul 04, 2019 at 12:34:30AM -0400, Joel Fernandes (Google) wrote:
> > > > > > It is possible that the rcuperf kernel test runs concurrently with init
> > > > > > starting up.  During this time, the system is running all grace periods
> > > > > > as expedited.  However, rcuperf can also be run for normal GP tests.
> > > > > > Right now, it depends on a holdoff time before starting the test to
> > > > > > ensure grace periods start later. This works fine with the default
> > > > > > holdoff time however it is not robust in situations where init takes
> > > > > > greater than the holdoff time to finish running. Or, as in my case:
> > > > > > 
> > > > > > I modified the rcuperf test locally to also run a thread that did
> > > > > > preempt disable/enable in a loop. This had the effect of slowing down
> > > > > > init. The end result was that the "batches:" counter in rcuperf was 0
> > > > > > causing a division by 0 error in the results. This counter was 0 because
> > > > > > only expedited GPs seem to happen, not normal ones which led to the
> > > > > > rcu_state.gp_seq counter remaining constant across grace periods which
> > > > > > unexpectedly happen to be expedited. The system was running expedited
> > > > > > RCU all the time because rcu_unexpedited_gp() would not have run yet
> > > > > > from init.  In other words, the test would concurrently with init
> > > > > > booting in expedited GP mode.
> > > > > > 
> > > > > > To fix this properly, let us check if system_state if SYSTEM_RUNNING
> > > > > > is set before starting the test. The system_state approximately aligns
> > > > 
> > > > Just minor typo..
> > > > 
> > > > To fix this properly, let us check if system_state if SYSTEM_RUNNING
> > > > is set before starting the test. ...
> > > > 
> > > > Should be
> > > > 
> > > > To fix this properly, let us check if system_state is set to
> > > > SYSTEM_RUNNING before starting the test. ...
> > > 
> > > That's a fair point. I wonder if Paul already fixed it up in his tree,
> > > however I am happy to resend if he hasn't. Paul, how would you like to handle
> > > this commit log nit?
> > > 
> > > it is just 'if ..' to 'is SYSTEM_RUNNING'
> > 
> > It now reads as follows:
> > 
> > 	To fix this properly, this commit waits until system_state is
> > 	set to SYSTEM_RUNNING before starting the test.  This change is
> > 	made just before kernel_init() invokes rcu_end_inkernel_boot(),
> > 	and this latter is what turns off boot-time expediting of RCU
> > 	grace periods.
> 
> Ok, looks good to me, thanks.
> 
> And for below patch,
> 
> Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>

Applied, thank you!

							Thnax, Paul

> > I dropped the last paragraph about late_initcall().  And I suspect that
> > the last clause from rcu_gp_is_expedited() can be dropped:
> > 
> > bool rcu_gp_is_expedited(void)
> > {
> > 	return rcu_expedited || atomic_read(&rcu_expedited_nesting) ||
> > 	       rcu_scheduler_active == RCU_SCHEDULER_INIT;
> > }
> > 
> > This is because rcu_expedited_nesting is initialized to 1, and is
> > decremented in rcu_end_inkernel_boot(), which is called long after
> > rcu_scheduler_active has been set to RCU_SCHEDULER_RUNNING, which
> > happens at core_initcall() time.  So if the last clause says "true",
> > so does the second-to-last clause.
> > 
> > The similar check in rcu_gp_is_normal() is still need, however, to allow
> > the power-management subsystem to invoke synchronize_rcu() just after
> > the scheduler has been initialized, but before RCU is aware of this.
> > 
> > So, how about the commit shown below?
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > commit 1f7e72efe3c761c2b34da7b59e01ad69c657db10
> > Author: Paul E. McKenney <paulmck@...ux.ibm.com>
> > Date:   Fri Jul 5 08:05:10 2019 -0700
> > 
> >     rcu: Remove redundant "if" condition from rcu_gp_is_expedited()
> >     
> >     Because rcu_expedited_nesting is initialized to 1 and not decremented
> >     until just before init is spawned, rcu_expedited_nesting is guaranteed
> >     to be non-zero whenever rcu_scheduler_active == RCU_SCHEDULER_INIT.
> >     This commit therefore removes this redundant "if" equality test.
> >     
> >     Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
> > 
> > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> > index 249517058b13..64e9cc8609e7 100644
> > --- a/kernel/rcu/update.c
> > +++ b/kernel/rcu/update.c
> > @@ -136,8 +136,7 @@ static atomic_t rcu_expedited_nesting = ATOMIC_INIT(1);
> >   */
> >  bool rcu_gp_is_expedited(void)
> >  {
> > -	return rcu_expedited || atomic_read(&rcu_expedited_nesting) ||
> > -	       rcu_scheduler_active == RCU_SCHEDULER_INIT;
> > +	return rcu_expedited || atomic_read(&rcu_expedited_nesting);
> >  }
> >  EXPORT_SYMBOL_GPL(rcu_gp_is_expedited);
> >  
> > 
> 

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