Re: Real-Time Preemption and RCU

[Esben Nielsen]

On Fri, 18 Mar 2005, Ingo Molnar wrote:

> 
> * Esben Nielsen <simlo@xxxxxxxxxx> wrote:
> 
> > Why can should there only be one RCU-reader per CPU at each given
> > instance? Even on a real-time UP system it would be very helpfull to
> > have RCU areas to be enterable by several tasks as once. It would
> > perform better, both wrt. latencies and throughput: With the above
> > implementation an high priority task entering an RCU area will have to
> > boost the current RCU reader, make a task switch until that one
> > finishes and makes yet another task switch. to get back to the high
> > priority task. With an RCU implementation which can take n RCU readers
> > per CPU there is no such problem.
> 
> correct, for RCU we could allow multiple readers per lock, because the
> 'blocking' side of RCU (callback processing) is never (supposed to be)
> in any latency path.
> 
> except if someone wants to make RCU callback processing deterministic at
> some point. (e.g. for memory management reasons.)

I think it can be deterministic (on the long timescale of memory management) 
anyway: Boost any non-RT task entering an RCU region to the lowest RT priority.
This way only all the RT tasks + one non-RT task can be within those
regions. The RT-tasks are supposed to have some kind of upper bound to
their CPU-usage. The non-RT task will also finish "soon" as it is
boosted. If the RCU batches are also at the lowest RT-priority they can be
run immediately after the non-RT task is done.

> 
> clearly the simplest solution is to go with the single-reader locks for
> now - a separate experiment could be done with a new type of rwlock that
> can only be used by the RCU code. (I'm not quite sure whether we could
> guarantee a minimum rate of RCU callback processing under such a scheme
> though. It's an eventual memory DoS otherwise.)
> 

Why are a lock needed at all? If it is doable without locking for an
non-preemptable SMP kernel it must be doable for an preemptable kernel as
well.I am convinced some kind of per-CPU rcu_read_count as I specified in
my previous mail can work some way or the other. call_rcu() might need to
do more complicated stuff and thus use CPU but call_rcu() is supposed to
be an relative rare event not worth optimizing for.  Such an
implementation will work for any preemptable kernel, not only PREEMPT_RT. 
For performance is considered it is important not to acquire any locks in
the rcu-read regions. 

I tried this approach. My UP labtop did boot on it, but I haven't testet
it further. I have included the very small patch as an attachment.

> 	Ingo

I have not yet looked at -V0.7.41-00...

Esben

diff -Naur --exclude-from diff_exclude linux-2.6.11-final-V0.7.40-00/include/linux/rcupdate.h linux-2.6.11-final-V0.7.40-00-RCU/include/linux/rcupdate.h
--- linux-2.6.11-final-V0.7.40-00/include/linux/rcupdate.h	2005-03-11 23:40:13.000000000 +0100
+++ linux-2.6.11-final-V0.7.40-00-RCU/include/linux/rcupdate.h	2005-03-19 12:47:09.000000000 +0100
@@ -85,6 +85,7 @@
  * curlist - current batch for which quiescent cycle started if any
  */
 struct rcu_data {
+	long            active_readers;
 	/* 1) quiescent state handling : */
 	long		quiescbatch;     /* Batch # for grace period */
 	int		passed_quiesc;	 /* User-mode/idle loop etc. */
@@ -115,12 +116,14 @@
 static inline void rcu_qsctr_inc(int cpu)
 {
 	struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
-	rdp->passed_quiesc = 1;
+	if(rdp->active_readers==0)
+		rdp->passed_quiesc = 1;
 }
 static inline void rcu_bh_qsctr_inc(int cpu)
 {
 	struct rcu_data *rdp = &per_cpu(rcu_bh_data, cpu);
-	rdp->passed_quiesc = 1;
+	if(rdp->active_readers==0)
+		rdp->passed_quiesc = 1;
 }
 
 static inline int __rcu_pending(struct rcu_ctrlblk *rcp,
@@ -183,29 +186,27 @@
  *
  * It is illegal to block while in an RCU read-side critical section.
  */
-#define rcu_read_lock()		preempt_disable()
+static inline void rcu_read_lock(void)
+{	
+	preempt_disable(); 
+	__get_cpu_var(rcu_data).active_readers++;
+	preempt_enable();
+}
 
 /**
  * rcu_read_unlock - marks the end of an RCU read-side critical section.
  *
  * See rcu_read_lock() for more information.
  */
-#define rcu_read_unlock()	preempt_enable()
+static inline void rcu_read_unlock(void)
+{	
+	preempt_disable(); 
+	__get_cpu_var(rcu_data).active_readers--;
+	preempt_enable();
+}
 
 #define IGNORE_LOCK(op, lock)	do { (void)(lock); op(); } while (0)
 
-#ifdef CONFIG_PREEMPT_RT
-# define rcu_read_lock_spin(lock)	spin_lock(lock)
-# define rcu_read_unlock_spin(lock)	spin_unlock(lock)
-# define rcu_read_lock_read(lock)	read_lock(lock)
-# define rcu_read_unlock_read(lock)	read_unlock(lock)
-# define rcu_read_lock_bh_read(lock)	read_lock_bh(lock)
-# define rcu_read_unlock_bh_read(lock)	read_unlock_bh(lock)
-# define rcu_read_lock_down_read(rwsem)	down_read(rwsem)
-# define rcu_read_unlock_up_read(rwsem)	up_read(rwsem)
-# define rcu_read_lock_nort()		do { } while (0)
-# define rcu_read_unlock_nort()		do { } while (0)
-#else
 # define rcu_read_lock_spin(lock)	IGNORE_LOCK(rcu_read_lock, lock)
 # define rcu_read_unlock_spin(lock)	IGNORE_LOCK(rcu_read_unlock, lock)
 # define rcu_read_lock_read(lock)	IGNORE_LOCK(rcu_read_lock, lock)
@@ -216,15 +217,10 @@
 # define rcu_read_unlock_nort()		rcu_read_unlock()
 # define rcu_read_lock_bh_read(lock)	IGNORE_LOCK(rcu_read_lock_bh, lock)
 # define rcu_read_unlock_bh_read(lock)	IGNORE_LOCK(rcu_read_unlock_bh, lock)
-#endif
 
-#ifdef CONFIG_PREEMPT_RT
-# define rcu_read_lock_sem(lock)	down(lock)
-# define rcu_read_unlock_sem(lock)	up(lock)
-#else
 # define rcu_read_lock_sem(lock)	IGNORE_LOCK(rcu_read_lock, lock)
 # define rcu_read_unlock_sem(lock)	IGNORE_LOCK(rcu_read_unlock, lock)
-#endif
+
 /*
  * So where is rcu_write_lock()?  It does not exist, as there is no
  * way for writers to lock out RCU readers.  This is a feature, not
diff -Naur --exclude-from diff_exclude linux-2.6.11-final-V0.7.40-00/Makefile linux-2.6.11-final-V0.7.40-00-RCU/Makefile
--- linux-2.6.11-final-V0.7.40-00/Makefile	2005-03-11 23:40:13.000000000 +0100
+++ linux-2.6.11-final-V0.7.40-00-RCU/Makefile	2005-03-19 12:41:09.000000000 +0100
@@ -1,7 +1,7 @@
 VERSION = 2
 PATCHLEVEL = 6
 SUBLEVEL = 11
-EXTRAVERSION = -RT-V0.7.40-00
+EXTRAVERSION = -RT-V0.7.40-00-RCU
 NAME=Woozy Numbat
 
 # *DOCUMENTATION*