Re: Real-Time Preemption and RCU

[Paul E. McKenney]

On Sun, Mar 20, 2005 at 02:29:17PM +0100, Esben Nielsen wrote:
> 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.

Hmmm...  Sort of a preemptive-first-strike priority boost.  Cute!  ;-)

> > 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. 

You definitely don't need a lock -- you can just suppress preemption
on the read side instead.  But that potentially makes for long scheduling
latencies.

The counter approach might work, and is also what the implementation #5
does -- check out rcu_read_lock() in Ingo's most recent patch.

						Thanx, Paul

> 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*

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