Date:	Tue, 8 Apr 2008 09:59:36 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, mathieu.desnoyers@...ymtl.ca,
	mingo@...e.hu, hch@...radead.org, mmlnx@...ibm.com,
	dipankar@...ibm.com, dsmith@...hat.com, rostedt@...dmis.org,
	adrian.bunk@...ial.fi, a.p.zijlstra@...llo.nl, ego@...ibm.com,
	niv@...ibm.com, dvhltc@...ibm.com, rusty@....ibm.com,
	jkenisto@...ux.vnet.ibm.com, oleg@...sign.ru
Subject: Re: [PATCH,RFC] Add call_rcu_sched()

On Tue, Apr 08, 2008 at 12:34:49AM -0700, Andrew Morton wrote:
> On Sun, 6 Apr 2008 14:37:19 -0700 "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > Hello!
> > 
> > Third cut of patch to provide the call_rcu_sched().  This is again to
> > synchronize_sched() as call_rcu() is to synchronize_rcu().
> > 
> > Should be fine for experimental use, but not ready for inclusion.
> 
> Let me know when to come out of hiding ;)

;-)

> > Passes multi-hour rcutorture sessions with concurrent CPU hotplugging.
> > 
> > Fixes since the first version include a bug that could result in
> > indefinite blocking (spotted by Gautham Shenoy), better resiliency
> > against CPU-hotplug operations, and other minor fixes.
> > 
> > Fixes since the second version include reworking grace-period detection
> > to avoid deadlocks that could happen when running concurrently with
> > CPU hotplug, adding Mathieu's fix to avoid the softlockup messages,
> > as well as Mathieu's fix to allow use earlier in boot.
> > 
> > Known/suspected shortcomings:
> > 
> > o	Only moderately tested on x86-64 and POWER -- a few hours of
> > 	rcutorture with concurrent CPU hotplugging.  In particular, I
> > 	still do not trust the sleep/wakeup logic between call_rcu_sched()
> > 	and rcu_sched_grace_period().
> > 
> > o	Need to add call_rcu_sched() testing to rcutorture.
> > 
> > o	Still needs rcu_barrier_sched() -- intending to incorporate
> > 	the version Mathieu provided.
> > 
> > This patch also fixes a long-standing bug in the earlier preemptable-RCU
> > implementation of synchronize_rcu() that could result in loss of
> > concurrent external changes to a task's CPU affinity mask.  I still cannot
> > remember who reported this...
> >
> > ...
> >
> > +#define call_rcu_sched(head, func) call_rcu(head, func)
> > +
> >  extern void __rcu_init(void);
> > +#define rcu_init_sched()	do { } while (0)
> 
> There are lots of creepy macros-which-probably-dont-need-to-be-macros in
> here.
> 
> > +
> > +static inline int
> > +rcu_qsctr_inc_needed_dyntick(int cpu)
> 
> Unneeded newline.

25-year-old habits die hard.  Fixed!

> > +{
> > +	long curr;
> > +	long snap;
> > +	struct rcu_dyntick_sched *rdssp = &per_cpu(rcu_dyntick_sched, cpu);
> > +
> > +	curr = rdssp->dynticks;
> > +	snap = rdssp->sched_dynticks_snap;
> > +	smp_mb(); /* force ordering with cpu entering/leaving dynticks. */
> > +
> > +	/*
> > +	 * If the CPU remained in dynticks mode for the entire time
> > +	 * and didn't take any interrupts, NMIs, SMIs, or whatever,
> > +	 * then it cannot be in the middle of an rcu_read_lock(), so
> > +	 * the next rcu_read_lock() it executes must use the new value
> > +	 * of the counter.  Therefore, this CPU has been in a quiescent
> > +	 * state the entire time, and we don't need to wait for it.
> > +	 */
> > +
> > +	if ((curr == snap) && ((curr & 0x1) == 0))
> > +		return 0;
> > +
> > +	/*
> > +	 * If the CPU passed through or entered a dynticks idle phase with
> > +	 * no active irq handlers, then, as above, this CPU has already
> > +	 * passed through a quiescent state.
> > +	 */
> > +
> > +	if ((curr - snap) > 2 || (snap & 0x1) == 0)
> > +		return 0;
> > +
> > +	/* We need this CPU to go through a quiescent state. */
> > +
> > +	return 1;
> > +}
> 
> That's a pretty big inline.  It only has a single callsite so the compiler
> should inline it for us.  And if it grows a second callsite, the inlining
> is probably wrong.

K, removed the "inline".  Though it is not all that big if comments are
removed.  ;-)

> > +static inline int
> > +rcu_qsctr_inc_needed(int cpu)
> 
> Unneeded newline.

Fixed.

> >  /*
> >   * Get here when RCU is idle.  Decide whether we need to
> >   * move out of idle state, and return non-zero if so.
> > @@ -821,6 +924,13 @@ void rcu_check_callbacks(int cpu, int us
> >  	unsigned long flags;
> >  	struct rcu_data *rdp = RCU_DATA_CPU(cpu);
> >  
> > +	if (user ||
> > +	    (idle_cpu(cpu) && !in_softirq() &&
> > +	     hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
> 
> I think this test could do with a bigfatcomment explaining what it is doing.

How about the following placed before the "if"?

	/*
	 * If this CPU took its interrupt from user mode or from the
	 * idle loop, and this is not a nested interrupt, then
	 * this CPU has to have exited all prior preept-disable
	 * sections of code.  So increment the counter to note this.
	 *
	 * The memory barrier is needed to handle the case where
	 * writes from a preempt-disable section of code gets reordered
	 * into schedule() by this CPU's write buffer.  So the memory
	 * barrier makes sure that the rcu_qsctr_inc() is seen by other
	 * CPUs to happen after any such write.
	 */

And I guess rcuclassic.c needs a similar comment in its version of
rcu_check_callbacks().  I will add this in a separate patch in this
set of patches.

> > +		smp_mb();	/* Guard against aggressive schedule(). */
> > +	     	rcu_qsctr_inc(cpu);
> > +	}
> > +
> >  	rcu_check_mb(cpu);
> >  	if (rcu_ctrlblk.completed == rdp->completed)
> >  		rcu_try_flip();
> >
> > ...
> >
> > +
> > +	/*
> > +	 * The rcu_sched grace-period processing might have bypassed
> > +	 * this CPU, given that it was not in the rcu_cpu_online_map
> > +	 * when the grace-period scan started.  This means that the
> > +	 * grace-period task might sleep.  So make sure that if this
> > +	 * should happen, the first callback posted to this CPU will
> > +	 * wake up the grace-period task if need be.
> > +	 */
> > +
> > +	local_irq_save(flags);
> > +	rdp = RCU_DATA_ME();
> > +	spin_lock(&rdp->lock);
> 
> I assume that splitting the irq-disable from the spin_lock is a little
> latency optimisation?

As Gautham pointed out, we cannot allow the task to be preempted between
the time that we pick up rdp and the time that we acquire rdp->lock.

Hmmm...  Wait a minute... This should set "cpu"'s rdp->rcu_sched_sleeping
to 1, not some random CPU's.  Guess I should fix that, thank you!!!

The new code is as follows:

	rdp = RCU_DATA_CPU(cpu);
	spin_lock_irqsave(&rdp->lock, flags);
	rdp->rcu_sched_sleeping = 1;
	spin_unlock_irqrestore(&rdp->lock, flags);

> > +	rdp->rcu_sched_sleeping = 1;
> > +	spin_unlock_irqrestore(&rdp->lock, flags);
> >  }
> >  
> >  #else /* #ifdef CONFIG_HOTPLUG_CPU */
> > @@ -993,26 +1129,194 @@ void call_rcu(struct rcu_head *head, voi
> >  }
> >  EXPORT_SYMBOL_GPL(call_rcu);
> >  
> > +void call_rcu_sched(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
> > +{
> > +	unsigned long flags;
> > +	struct rcu_data *rdp;
> > +	int wake_gp = 0;
> > +
> > +	head->func = func;
> > +	head->next = NULL;
> > +	local_irq_save(flags);
> > +	rdp = RCU_DATA_ME();
> > +	spin_lock(&rdp->lock);
> > +	*rdp->nextschedtail = head;
> > +	rdp->nextschedtail = &head->next;
> > +	if (rdp->rcu_sched_sleeping) {
> > +
> > +		/* Grace-period processing might be sleeping... */
> > +
> > +		rdp->rcu_sched_sleeping = 0;
> > +		wake_gp = 1;
> > +	}
> > +	spin_unlock(&rdp->lock);
> > +	local_irq_restore(flags);
> 
> spin_unlock_irqrestore() here would be consistent with the above.

One line less code as well.  I fixed call_rcu() as well.

> > +	if (wake_gp) {
> > +
> > +		/* Wake up grace-period processing, unless someone beat us. */
> > +
> > +		spin_lock_irqsave(&rcu_ctrlblk.schedlock, flags);
> 
> If wake_gp!=0 is common then we could microoptimise straight-line
> performance here by retaining the irq-offness from above.

If wake_gp!=0 is common, then that means that this section of code
is rarely invoked.  In which case I would guess that we should avoid
the optimization, correct?

> > +		if (rcu_ctrlblk.sched_sleep != rcu_sched_sleeping)
> > +			wake_gp = 0;
> > +		rcu_ctrlblk.sched_sleep = rcu_sched_not_sleeping;
> > +		spin_unlock_irqrestore(&rcu_ctrlblk.schedlock, flags);
> > +		if (wake_gp)
> > +			wake_up_interruptible(&rcu_ctrlblk.sched_wq);
> > +	}
> > +}
> > +EXPORT_SYMBOL_GPL(call_rcu_sched);
> >
> > ...
> >
> > +static int
> > +rcu_sched_grace_period(void *arg)
> 
> Unneeded newline.

Fixed.

> >  {
> > -	cpumask_t oldmask;
> > +	int couldsleep;		/* might sleep after current pass. */
> > +	int couldsleepnext = 0; /* might sleep after next pass. */
> >  	int cpu;
> > +	unsigned long flags;
> > +	struct rcu_data *rdp;
> > +	int ret;
> >  
> > -	if (sched_getaffinity(0, &oldmask) < 0)
> > -		oldmask = cpu_possible_map;
> > -	for_each_online_cpu(cpu) {
> > -		sched_setaffinity(0, cpumask_of_cpu(cpu));
> > -		schedule();
> > -	}
> > -	sched_setaffinity(0, oldmask);
> > +	/*
> > +	 * Each pass through the following loop handles one
> > +	 * rcu_sched grace period cycle.
> > +	 */
> > +
> > +	do {
> > +		
> > +		/* Save each CPU's current state. */
> > +
> > +		for_each_online_cpu(cpu) {
> 
> Numerous unneeded newline ;)

OK.  I am guessing the one after the "do".  The others are the ones
following each comment block?

> > +			dyntick_save_progress_counter_sched(cpu);
> > +			save_qsctr_sched(cpu);
> > +		}
> > +
> > +		/*
> > +		 * Sleep for about an RCU grace-period's worth to
> > +		 * allow better batching and to consume less CPU.
> > +		 */
> > +
> > +		schedule_timeout_interruptible(HZ / 20);
> 
> eek, a magic number.

Added a cpp macro RCU_SCHED_BATCH_TIME defined to be HZ / 20.  ;-)

> > +		/*
> > +		 * If there was nothing to do last time, prepare to
> > +		 * sleep at the end of the current grace period cycle.
> > +		 */
> > +
> > +		couldsleep = couldsleepnext;
> > +		couldsleepnext = 1;
> > +		if (couldsleep) {
> > +			spin_lock_irqsave(&rcu_ctrlblk.schedlock, flags);
> > +			rcu_ctrlblk.sched_sleep = rcu_sched_sleep_prep;
> > +			spin_unlock_irqrestore(&rcu_ctrlblk.schedlock, flags);
> > +		}
> 
> If the above locking actually correct and needed?  The write to
> rcu_ctrlblk.sched_sleep is a single word...

Yes, but the code sequence in call_rcu_sched() that is protected by
the same lock would be shocked and disappointed if that write showed
up in the middle of its critical section.  ;-)

> > +		/*
> > +		 * Wait on each CPU in turn to have either visited
> > +		 * a quiescent state or been in dynticks-idle mode.
> > +		 */
> > +
> > +		for_each_online_cpu(cpu) {
> > +			while (rcu_qsctr_inc_needed(cpu) &&
> > +			       rcu_qsctr_inc_needed_dyntick(cpu)) {
> > +				/* resched_cpu(cpu); */
> > +				schedule_timeout_interruptible(1);
> > +			}
> > +		}
> > +
> > +		/*
> > +		 * Advance callbacks for each CPU.
> > +		 */
> > +
> > +		for_each_online_cpu(cpu) {
> 
> It's more conventional to omit the blank line after the above form of
> comment block.

Also no point in its being a multi-line comment block.  Changed to
single-line comment block.  And deleted the blank lines following
each multi-line comment block.  More 25-year-old habits...

> > +			rdp = RCU_DATA_CPU(cpu);
> > +			spin_lock_irqsave(&rdp->lock, flags);
> > +
> > +			/*
> > +			 * We are running on this CPU irq-disabled, so no
> > +			 * CPU can go offline until we re-enable irqs.
> 
> but, but, but.  The cpu at `cpu' could have gone offline just before we
> disabled local interrupts.

Indeed.  I added the following to the comment:

			 * The current CPU might have already gone
			 * offline (between the for_each_offline_cpu and
			 * the spin_lock_irqsave), but in that case all its
			 * callback lists will be empty, so no harm done.

Good catch!!!

> > +			 * Advance the callbacks!  We share normal RCU's
> > +			 * donelist, since callbacks are invoked the
> > +			 * same way in either case.
> > +			 */
> > +
> > +			if (rdp->waitschedlist != NULL) {
> > +				*rdp->donetail = rdp->waitschedlist;
> > +				rdp->donetail = rdp->waitschedtail;
> > +
> > +				/*
> > +				 * Next rcu_check_callbacks() will
> > +				 * do the required raise_softirq().
> > +				 */
> > +			}
> > +			if (rdp->nextschedlist != NULL) {
> > +				rdp->waitschedlist = rdp->nextschedlist;
> > +				rdp->waitschedtail = rdp->nextschedtail;
> > +				couldsleep = 0;
> > +				couldsleepnext = 0;
> > +			} else {
> > +				rdp->waitschedlist = NULL;
> > +				rdp->waitschedtail = &rdp->waitschedlist;
> > +			}
> > +			rdp->nextschedlist = NULL;
> > +			rdp->nextschedtail = &rdp->nextschedlist;
> > +
> > +			/* Mark sleep intention. */
> > +
> > +			rdp->rcu_sched_sleeping = couldsleep;
> > +
> > +			spin_unlock_irqrestore(&rdp->lock, flags);
> > +		}
> > +
> > +		/* If we saw callbacks on the last scan, go deal with them. */
> > +
> > +		if (!couldsleep)
> > +			continue;
> > +
> > +		/* Attempt to block... */
> > +
> > +		spin_lock_irqsave(&rcu_ctrlblk.schedlock, flags);
> > +		if (rcu_ctrlblk.sched_sleep != rcu_sched_sleep_prep) {
> > +
> > +			/*
> > +			 * Someone posted a callback after we scanned.
> > +			 * Go take care of it.
> > +			 */
> > +
> > +			spin_unlock_irqrestore(&rcu_ctrlblk.schedlock, flags);
> > +			couldsleepnext = 0;
> > +			continue;
> > +		}
> > +
> > +		/* Block until the next person posts a callback. */
> > +
> > +		rcu_ctrlblk.sched_sleep = rcu_sched_sleeping;
> > +		spin_unlock_irqrestore(&rcu_ctrlblk.schedlock, flags);
> > +		ret = 0;
> > +		__wait_event_interruptible(rcu_ctrlblk.sched_wq,
> > +			rcu_ctrlblk.sched_sleep != rcu_sched_sleeping,
> > +			ret);
> > +		if (ret)
> > +			flush_signals(current);
> 
> That flush_signals() was a surprise.  A desurprising comment would be nice.

I added the following comment:

		/*
		 * Signals would prevent us from sleeping, and we cannot
		 * do much with them in any case.  So flush them.
		 */

Seem reasonable?

> > +		couldsleepnext = 0;
> > +
> > +	} while (!kthread_should_stop());
> > +
> > +	return (0);
> >  }
> > -EXPORT_SYMBOL_GPL(__synchronize_sched);
> >  
> >  /*
> >   * Check to see if any future RCU-related work will need to be done
> > @@ -1029,7 +1333,9 @@ int rcu_needs_cpu(int cpu)
> >  
> >  	return (rdp->donelist != NULL ||
> >  		!!rdp->waitlistcount ||
> > -		rdp->nextlist != NULL);
> > +		rdp->nextlist != NULL ||
> > +		rdp->nextschedlist != NULL ||
> > +		rdp->waitschedlist != NULL);
> >  }
> >  
> >  int rcu_pending(int cpu)
> > @@ -1040,7 +1346,9 @@ int rcu_pending(int cpu)
> >  
> >  	if (rdp->donelist != NULL ||
> >  	    !!rdp->waitlistcount ||
> > -	    rdp->nextlist != NULL)
> > +	    rdp->nextlist != NULL ||
> > +	    rdp->nextschedlist != NULL ||
> > +	    rdp->waitschedlist != NULL)
> >  		return 1;
> >  
> >  	/* The RCU core needs an acknowledgement from this CPU. */
> > @@ -1107,6 +1415,11 @@ void __init __rcu_init(void)
> >  		rdp->donetail = &rdp->donelist;
> >  		rdp->rcu_flipctr[0] = 0;
> >  		rdp->rcu_flipctr[1] = 0;
> > +		rdp->nextschedlist = NULL;
> > +		rdp->nextschedtail = &rdp->nextschedlist;
> > +		rdp->waitschedlist = NULL;
> > +		rdp->waitschedtail = &rdp->waitschedlist;
> > +		rdp->rcu_sched_sleeping = 0;
> >  	}
> >  	register_cpu_notifier(&rcu_nb);
> >  
> > @@ -1129,6 +1442,18 @@ void __init __rcu_init(void)
> >  }
> >  
> >  /*
> > + * Late-boot-time RCU initialization that must wait until after scheduler
> > + * has been initialized.
> > + */
> > +void __init rcu_init_sched(void)
> > +{
> > +	rcu_sched_grace_period_task = kthread_run(rcu_sched_grace_period,
> > +						  NULL,
> > +						  "rcu_sched_grace_period");
> > +	WARN_ON(IS_ERR(rcu_sched_grace_period_task));
> > +}
> > +
> > +/*
> >   * Deprecated, use synchronize_rcu() or synchronize_sched() instead.
> >   */
> >  void synchronize_kernel(void)
> 
> I suspect I don't understand any of the RCU code any more.

This definition for synchronize_kernel() somehow slipped back in.
I have removed it.  Sorry for the confusion!!!

							Thanx, Paul
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