Re: [PATCH RFC] doc: rcu: remove obsolete (non-)requirement about disabling preemption

[Joel Fernandes <joel@joelfernandes.org>]

On Wed, Oct 17, 2018 at 01:33:24PM -0700, Paul E. McKenney wrote:
> On Wed, Oct 17, 2018 at 11:15:05AM -0700, Joel Fernandes wrote:
> > On Wed, Oct 17, 2018 at 09:11:00AM -0700, Paul E. McKenney wrote:
> > > On Tue, Oct 16, 2018 at 01:41:22PM -0700, Joel Fernandes wrote:
> > > > On Tue, Oct 16, 2018 at 04:26:11AM -0700, Paul E. McKenney wrote:
> > > > > On Mon, Oct 15, 2018 at 02:08:56PM -0700, Paul E. McKenney wrote:
> > > > > > On Mon, Oct 15, 2018 at 01:15:56PM -0700, Joel Fernandes wrote:
> > > > > > > On Mon, Oct 15, 2018 at 12:54:26PM -0700, Paul E. McKenney wrote:
> > > > > > > [...]
> > > > > > > > > > In any case, please don't spin for milliseconds with preemption disabled.
> > > > > > > > > > The real-time guys are unlikely to be happy with you if you do this!
> > > > > > > > > 
> > > > > > > > > Well just to clarify, I was just running Oleg's test which did this. This
> > > > > > > > > test was mentioned in the original documentation that I deleted. Ofcourse I
> > > > > > > > > would not dare do such a thing in production code :-D. I guess to Oleg's
> > > > > > > > > defense, he did it to very that synchronize_rcu() was not blocked on
> > > > > > > > > preempt-disable sections which was a different test.
> > > > > > > > 
> > > > > > > > Understood!  Just pointing out that RCU's tolerating a given action does
> > > > > > > > not necessarily mean that it is a good idea to take that action.  ;-)
> > > > > > > 
> > > > > > > Makes sense :-) thanks.
> > > > > > 
> > > > > > Don't worry, that won't happen again.  ;-)
> > > > > > 
> > > > > > > > > > > > +		pr_crit("SPIN done!\n");
> > > > > > > > > > > > +		preempt_enable();
> > > > > > > > > > > > +		break;
> > > > > > > > > > > > +	case 777:
> > > > > > > > > > > > +		pr_crit("SYNC start\n");
> > > > > > > > > > > > +		synchronize_rcu();
> > > > > > > > > > > > +		pr_crit("SYNC done!\n");
> > > > > > > > > > > 
> > > > > > > > > > > But you are using the console printing infrastructure which is rather
> > > > > > > > > > > heavyweight. Try replacing pr_* calls with trace_printk so that you
> > > > > > > > > > > write to the lock-free ring buffer, this will reduce the noise from the
> > > > > > > > > > > heavy console printing infrastructure.
> > > > > > > > > > 
> > > > > > > > > > And this might be a problem as well.
> > > > > > > > > 
> > > > > > > > > This was not the issue (or atleast not fully the issue) since I saw the same
> > > > > > > > > thing with trace_printk. It was exactly what you said - which is the
> > > > > > > > > excessively long preempt disabled times.
> > > > > > > > 
> > > > > > > > One approach would be to apply this patch against (say) v4.18, which
> > > > > > > > does not have consolidated grace periods.  You might then be able to
> > > > > > > > tell if the pr_crit() calls make any difference.
> > > > > > > 
> > > > > > > I could do that, yeah. But since the original problem went away due to
> > > > > > > disabling preempts for a short while, I will move on and continue to focus on
> > > > > > > updating other parts of the documenation. Just to mention I
> > > > > > > brought this up because I thought its better to do that than not to, just
> > > > > > > incase there is any lurking issue with the consolidation. Sorry if that ended
> > > > > > > up with me being noisy.
> > > > > > 
> > > > > > Not a problem, no need to apologize!
> > > > > 
> > > > > Besides, digging through the code did point out a reasonable optimization.
> > > > > In the common case, this would buy 100s of microseconds rather than
> > > > > milliseconds, but it seems simple enough to be worthwhile.  Thoughts?
> > > > 
> > > > Cool, thanks. One comment below:
> > > > 
> > > > > ------------------------------------------------------------------------
> > > > > 
> > > > > commit 07921e8720907f58f82b142f2027fc56d5abdbfd
> > > > > Author: Paul E. McKenney <paulmck@linux.ibm.com>
> > > > > Date:   Tue Oct 16 04:12:58 2018 -0700
> > > > > 
> > > > >     rcu: Speed up expedited GPs when interrupting RCU reader
> > > > >     
> > > > >     In PREEMPT kernels, an expedited grace period might send an IPI to a
> > > > >     CPU that is executing an RCU read-side critical section.  In that case,
> > > > >     it would be nice if the rcu_read_unlock() directly interacted with the
> > > > >     RCU core code to immediately report the quiescent state.  And this does
> > > > >     happen in the case where the reader has been preempted.  But it would
> > > > >     also be a nice performance optimization if immediate reporting also
> > > > >     happened in the preemption-free case.
> > > > >     
> > > > >     This commit therefore adds an ->exp_hint field to the task_struct structure's
> > > > >     ->rcu_read_unlock_special field.  The IPI handler sets this hint when
> > > > >     it has interrupted an RCU read-side critical section, and this causes
> > > > >     the outermost rcu_read_unlock() call to invoke rcu_read_unlock_special(),
> > > > >     which, if preemption is enabled, reports the quiescent state immediately.
> > > > >     If preemption is disabled, then the report is required to be deferred
> > > > >     until preemption (or bottom halves or interrupts or whatever) is re-enabled.
> > > > >     
> > > > >     Because this is a hint, it does nothing for more complicated cases.  For
> > > > >     example, if the IPI interrupts an RCU reader, but interrupts are disabled
> > > > >     across the rcu_read_unlock(), but another rcu_read_lock() is executed
> > > > >     before interrupts are re-enabled, the hint will already have been cleared.
> > > > >     If you do crazy things like this, reporting will be deferred until some
> > > > >     later RCU_SOFTIRQ handler, context switch, cond_resched(), or similar.
> > > > >     
> > > > >     Reported-by: Joel Fernandes <joel@joelfernandes.org>
> > > > >     Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
> > > > > 
> > > > > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > > > > index 004ca21f7e80..64ce751b5fe9 100644
> > > > > --- a/include/linux/sched.h
> > > > > +++ b/include/linux/sched.h
> > > > > @@ -571,8 +571,10 @@ union rcu_special {
> > > > >  	struct {
> > > > >  		u8			blocked;
> > > > >  		u8			need_qs;
> > > > > +		u8			exp_hint; /* Hint for performance. */
> > > > > +		u8			pad; /* No garbage from compiler! */
> > > > >  	} b; /* Bits. */
> > > > > -	u16 s; /* Set of bits. */
> > > > > +	u32 s; /* Set of bits. */
> > > > >  };
> > > > >  
> > > > >  enum perf_event_task_context {
> > > > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> > > > > index e669ccf3751b..928fe5893a57 100644
> > > > > --- a/kernel/rcu/tree_exp.h
> > > > > +++ b/kernel/rcu/tree_exp.h
> > > > > @@ -692,8 +692,10 @@ static void sync_rcu_exp_handler(void *unused)
> > > > >  	 */
> > > > >  	if (t->rcu_read_lock_nesting > 0) {
> > > > >  		raw_spin_lock_irqsave_rcu_node(rnp, flags);
> > > > > -		if (rnp->expmask & rdp->grpmask)
> > > > > +		if (rnp->expmask & rdp->grpmask) {
> > > > >  			rdp->deferred_qs = true;
> > > > > +			WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
> > > > > +		}
> > > > >  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> > > > >  	}
> > > > >  
> > > > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > > > > index 8b48bb7c224c..d6286eb6e77e 100644
> > > > > --- a/kernel/rcu/tree_plugin.h
> > > > > +++ b/kernel/rcu/tree_plugin.h
> > > > > @@ -643,8 +643,9 @@ static void rcu_read_unlock_special(struct task_struct *t)
> > > > >  	local_irq_save(flags);
> > > > >  	irqs_were_disabled = irqs_disabled_flags(flags);
> > > > >  	if ((preempt_bh_were_disabled || irqs_were_disabled) &&
> > > > > -	    t->rcu_read_unlock_special.b.blocked) {
> > > > > +	    t->rcu_read_unlock_special.s) {
> > > > >  		/* Need to defer quiescent state until everything is enabled. */
> > > > > +		WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
> > > > >  		raise_softirq_irqoff(RCU_SOFTIRQ);
> > > > 
> > > > Still going through this patch, but it seems to me like the fact that
> > > > rcu_read_unlock_special is called means someone has requested for a grace
> > > > period. Then in that case, does it not make sense to raise the softirq
> > > > for processing anyway?
> > > 
> > > Not necessarily.  Another reason that rcu_read_unlock_special() might
> > > be called is if the RCU read-side critical section had been preempted,
> > > in which case there might not even be a grace period in progress.
> > 
> > Yes true, it was at the back of my head ;) It needs to remove itself from the
> > blocked lists on the unlock. And ofcourse the preemption case is alsoo
> > clearly mentioned in this function's comments. (slaps self).
> 
> Sometimes rcutorture reminds me of interesting RCU corner cases...  ;-)
> 
> > > In addition, if interrupts, bottom halves, and preemption are all enabled,
> > > the code in rcu_preempt_deferred_qs_irqrestore() doesn't need to bother
> > > raising softirq, as it can instead just immediately report the quiescent
> > > state.
> > 
> > Makes sense. I will go through these code paths more today. Thank you for the
> > explanations!
> > 
> > I think something like need_exp_qs instead of 'exp_hint' may be more
> > descriptive?
> 
> Well, it is only a hint due to the fact that it is not preserved across
> complex sequences of overlapping RCU read-side critical sections of
> different types.  So if you have the following sequence:
> 
> 	rcu_read_lock();
> 	/* Someone does synchronize_rcu_expedited(), which sets ->exp_hint. */
> 	preempt_disable();
> 	rcu_read_unlock(); /* Clears ->exp_hint. */
> 	preempt_enable(); /* But ->exp_hint is already cleared. */
> 
> This is OK because there will be some later event that passes the quiescent
> state to the RCU core.  This will slow down the expedited grace period,
> but this case should be uncommon.  If it does turn out to be common, then
> some more complex scheme can be put in place.
> 
> Hmmm...  This patch does need some help, doesn't it?  How about the following
> to be folded into the original?
> 
> commit d8d996385055d4708121fa253e04b4272119f5e2
> Author: Paul E. McKenney <paulmck@linux.ibm.com>
> Date:   Wed Oct 17 13:32:25 2018 -0700
> 
>     fixup! rcu: Speed up expedited GPs when interrupting RCU reader
>     
>     Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index d6286eb6e77e..117aeb582fdc 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -650,6 +650,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
>  		local_irq_restore(flags);
>  		return;
>  	}
> +	WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
>  	rcu_preempt_deferred_qs_irqrestore(t, flags);
>  }
>  

Sure, I believe so. I was also thinking out load about if we can avoid
raising of the softirq for some cases in rcu_read_unlock_special:

For example, in rcu_read_unlock_special()

static void rcu_read_unlock_special(struct task_struct *t)
{
[...]
	if ((preempt_bh_were_disabled || irqs_were_disabled) &&
	    t->rcu_read_unlock_special.s) {
		/* Need to defer quiescent state until everything is enabled. */
		raise_softirq_irqoff(RCU_SOFTIRQ);
		local_irq_restore(flags);
		return;
	}
	rcu_preempt_deferred_qs_irqrestore(t, flags);
}

Instead of raising the softirq, for the case where irqs are enabled, but
preemption is disabled, can we not just do:

		set_tsk_need_resched(current);
		set_preempt_need_resched();

and return? Not sure the benefits of doing that are, but it seems nice to
avoid raising the softirq if possible, for benefit of real-time workloads.

Also it seems like there is a chance the softirq might run before the
preemption is reenabled anyway right?

Also one last thing, in your patch - do we really need to test for
"t->rcu_read_unlock_special.s" in rcu_read_unlock_special()? AFAICT,
rcu_read_unlock_special would only be called if t->rcu_read_unlock_special.s
is set in the first place so we can drop the test for that.

thanks,

- Joel