Re: [PATCH RFC tip/core/rcu] rcu: direct algorithmic SRCU implementation

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Feb 15, 2012 at 01:59:23PM +0100, Peter Zijlstra wrote:
> On Sun, 2012-02-12 at 18:09 -0800, Paul E. McKenney wrote:
> > The current implementation of synchronize_srcu_expedited() can cause
> > severe OS jitter due to its use of synchronize_sched(), which in turn
> > invokes try_stop_cpus(), which causes each CPU to be sent an IPI.
> > This can result in severe performance degradation for real-time workloads
> > and especially for short-interation-length HPC workloads.  Furthermore,
> > because only one instance of try_stop_cpus() can be making forward progress
> > at a given time, only one instance of synchronize_srcu_expedited() can
> > make forward progress at a time, even if they are all operating on
> > distinct srcu_struct structures.
> > 
> > This commit, inspired by an earlier implementation by Peter Zijlstra
> > (https://lkml.org/lkml/2012/1/31/211) and by further offline discussions,
> > takes a strictly algorithmic bits-in-memory approach.  This has the
> > disadvantage of requiring one explicit memory-barrier instruction in
> > each of srcu_read_lock() and srcu_read_unlock(), but on the other hand
> > completely dispenses with OS jitter and furthermore allows SRCU to be
> > used freely by CPUs that RCU believes to be idle or offline.
> > 
> > The update-side implementation handles the single read-side memory
> > barrier by rechecking the per-CPU counters after summing them and
> > by running through the update-side state machine twice.
> 
> Yeah, getting rid of that second memory barrier in srcu_read_lock() is
> pure magic :-)
> 
> > This implementation has passed moderate rcutorture testing on both 32-bit
> > x86 and 64-bit Power.  A call_srcu() function will be present in a later
> > version of this patch.
> 
> Goodness ;-)

Glad you like the magic and the prospect of call_srcu().  ;-)

> > @@ -131,10 +214,11 @@ int __srcu_read_lock(struct srcu_struct *sp)
> >  	int idx;
> >  
> >  	preempt_disable();
> > -	idx = sp->completed & 0x1;
> > -	barrier();  /* ensure compiler looks -once- at sp->completed. */
> > -	per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]++;
> > -	srcu_barrier();  /* ensure compiler won't misorder critical section. */
> > +	idx = rcu_dereference_index_check(sp->completed,
> > +					  rcu_read_lock_sched_held()) & 0x1;
> > +	ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]) +=
> > +		SRCU_USAGE_COUNT + 1;
> > +	smp_mb(); /* B */  /* Avoid leaking the critical section. */
> >  	preempt_enable();
> >  	return idx;
> >  }
> 
> You could use __this_cpu_* muck to shorten some of that.

Ah, so something like this?

	ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) += 
		SRCU_USAGE_COUNT + 1;

Now that you mention it, this does look nicer, applied here and to
srcu_read_unlock().

> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>


							Thanx, Paul