Re: [RFC v1 1/2] rcu/tree: Clean up dynticks counter usage

[Joel Fernandes <joel@joelfernandes.org>]

On Wed, Aug 28, 2019 at 01:13:44PM -0700, Paul E. McKenney wrote:
> On Mon, Aug 26, 2019 at 09:33:53PM -0400, Joel Fernandes (Google) wrote:
> > The dynticks counter are confusing due to crowbar writes of
> > DYNTICK_IRQ_NONIDLE whose purpose is to detect half-interrupts (i.e. we
> > see rcu_irq_enter() but not rcu_irq_exit() due to a usermode upcall) and
> > if so then do a reset of the dyntick_nmi_nesting counters. This patch
> > tries to get rid of DYNTICK_IRQ_NONIDLE while still keeping the code
> > working, fully functional, and less confusing. The confusion recently
> > has even led to patches forgetting that DYNTICK_IRQ_NONIDLE was written
> > to which wasted lots of time.
> > 
> > The patch has the following changes:
[snip]
> >  /*
> >   * Grace-period counter management.
> >   */
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 68ebf0eb64c8..255cd6835526 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -81,7 +81,7 @@
> >  
> >  static DEFINE_PER_CPU_SHARED_ALIGNED(struct rcu_data, rcu_data) = {
> >  	.dynticks_nesting = 1,
> > -	.dynticks_nmi_nesting = DYNTICK_IRQ_NONIDLE,
> > +	.dynticks_nmi_nesting = 0,
> 
> C initializes to zero by default, so this can simply be deleted.

Fixed.

> >  	.dynticks = ATOMIC_INIT(RCU_DYNTICK_CTRL_CTR),
> >  };
> >  struct rcu_state rcu_state = {
> > @@ -558,17 +558,18 @@ EXPORT_SYMBOL_GPL(rcutorture_get_gp_data);
> >  /*
> >   * Enter an RCU extended quiescent state, which can be either the
> >   * idle loop or adaptive-tickless usermode execution.
> > - *
> > - * We crowbar the ->dynticks_nmi_nesting field to zero to allow for
> > - * the possibility of usermode upcalls having messed up our count
> > - * of interrupt nesting level during the prior busy period.
> >   */
> >  static void rcu_eqs_enter(bool user)
> >  {
> >  	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
> >  
> > -	WARN_ON_ONCE(rdp->dynticks_nmi_nesting != DYNTICK_IRQ_NONIDLE);
> > -	WRITE_ONCE(rdp->dynticks_nmi_nesting, 0);
> > +	/* Entering usermode/idle from interrupt is not handled. These would
> > +	 * mean usermode upcalls or idle entry happened from interrupts. But,
> > +	 * reset the counter if we warn.
> > +	 */
> 
> Please either put the "/*" on its own line or use "//"-style comments.

I'll put "/*" on its own line.

> >  	WARN_ON_ONCE(rdp->dynticks_nmi_nesting <= 0);
> >  	WARN_ON_ONCE(rcu_dynticks_curr_cpu_in_eqs());
> >  
> > +	WRITE_ONCE(rdp->dynticks_nmi_nesting, /* No store tearing. */
> > +		   rdp->dynticks_nmi_nesting - 1);
> 
> This is problematic.  The +/-1 and +/-2 dance is specifically for NMIs, so...

This counter is deleted in the following patch so I hope its Ok to leave it
here for this one. I just kept it split into different patch to make
testing/review/development easier.

> >  	if (irq)
> >  		rcu_prepare_for_idle();
> > @@ -723,10 +728,6 @@ void rcu_irq_exit_irqson(void)
> >  /*
> >   * Exit an RCU extended quiescent state, which can be either the
> >   * idle loop or adaptive-tickless usermode execution.
> > - *
> > - * We crowbar the ->dynticks_nmi_nesting field to DYNTICK_IRQ_NONIDLE to
> > - * allow for the possibility of usermode upcalls messing up our count of
> > - * interrupt nesting level during the busy period that is just now starting.
> >   */
> >  static void rcu_eqs_exit(bool user)
> >  {
> > @@ -747,8 +748,13 @@ static void rcu_eqs_exit(bool user)
> >  	trace_rcu_dyntick(TPS("End"), rdp->dynticks_nesting, 1, rdp->dynticks);
> >  	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
> >  	WRITE_ONCE(rdp->dynticks_nesting, 1);
> > -	WARN_ON_ONCE(rdp->dynticks_nmi_nesting);
> > -	WRITE_ONCE(rdp->dynticks_nmi_nesting, DYNTICK_IRQ_NONIDLE);
> > +
> > +	/* Exiting usermode/idle from interrupt is not handled. These would
> > +	 * mean usermode upcalls or idle exit happened from interrupts. But,
> > +	 * reset the counter if we warn.
> > +	 */
> > +	if (WARN_ON_ONCE(rdp->dynticks_nmi_nesting != 0))
> > +		WRITE_ONCE(rdp->dynticks_nmi_nesting, 0);
> 
> And here.  Plus this is adding a test and branch in the common case.
> Given that the location being written to should be hot in the cache,
> it is not clear that this is a win.

The next patch removes the branch itself and just has the warning.

> >  	WARN_ON_ONCE(rdp->dynticks_nmi_nesting < 0);
> >  
> >  	/*
> > @@ -826,16 +833,21 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
> >  
> >  		incby = 1;
> >  	} else if (tick_nohz_full_cpu(rdp->cpu) &&
> > -		   rdp->dynticks_nmi_nesting == DYNTICK_IRQ_NONIDLE &&
> > -		   rdp->rcu_urgent_qs && !rdp->rcu_forced_tick) {
> > +		   !rdp->dynticks_nmi_nesting && rdp->rcu_urgent_qs &&
> > +		   !rdp->rcu_forced_tick) {
> 
> OK.  Though you should be able to save a line by pulling the
> "rdp->rcu_urgent_qs &&" onto the first line.

Fixed.

> >  		rdp->rcu_forced_tick = true;
> >  		tick_dep_set_cpu(rdp->cpu, TICK_DEP_BIT_RCU);
> >  	}
> > +
> 
> Not clear that the added blank line is a win, here or below.

Fixed,

thanks!

 - Joel

[snip]