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

[Joel Fernandes <joel@joelfernandes.org>]

On Thu, Oct 18, 2018 at 07:46:37AM -0700, Paul E. McKenney wrote:
[..]
> > > > > > > ------------------------------------------------------------------------
> > > > > > > 
> > > > > > > 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.
> 
> This approach would work very well in the case when preemption or bottom
> halves were disabled, but would not handle the case where interrupts were
> enabled during the RCU read-side critical section, an expedited grace
> period started (thus setting ->exp_hint), interrupts where then disabled,
> and finally rcu_read_unlock() was invoked.  Re-enabling interrupts would
> not cause either softirq or the scheduler to do anything, so the end of
> the expedited grace period might be delayed for some time, for example,
> until the next scheduling-clock interrupt.
> 
> But please see below.
> 
> > Also it seems like there is a chance the softirq might run before the
> > preemption is reenabled anyway right?
> 
> Not unless the rcu_read_unlock() is invoked from within a softirq
> handler on the one hand or within an interrupt handler that interrupted
> a preempt-disable region of code.  Otherwise, because interrupts are
> disabled, the raise_softirq() will wake up ksoftirqd, which cannot run
> until both preemption and bottom halves are enabled.
> 
> > 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.
> 
> Good point!
> 
> How about the following?
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> static void rcu_read_unlock_special(struct task_struct *t)
> {
> 	unsigned long flags;
> 	bool preempt_bh_were_disabled =
> 			!!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK));
> 	bool irqs_were_disabled;
> 
> 	/* NMI handlers cannot block and cannot safely manipulate state. */
> 	if (in_nmi())
> 		return;
> 
> 	local_irq_save(flags);
> 	irqs_were_disabled = irqs_disabled_flags(flags);
> 	if (preempt_bh_were_disabled || irqs_were_disabled) {
> 		WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
> 		/* Need to defer quiescent state until everything is enabled. */
> 		if (irqs_were_disabled) {
> 			raise_softirq_irqoff(RCU_SOFTIRQ);
> 		} else {
> 			set_tsk_need_resched(current);
> 			set_preempt_need_resched();
> 		}

Looks good to me, thanks! Maybe some code comments would be nice as well.

Shouldn't we also set_tsk_need_resched for the irqs_were_disabled case, so
that say if we are in an IRQ disabled region (local_irq_disable), then
ksoftirqd would run as possible once IRQs are renabled?

By the way, the user calling preempt_enable_no_resched would be another case
where the expedited grace period might extend longer than needed with the
above patch, but that seems unlikely enough to worry about :-)

thanks,

- Joel