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

[Mathieu Desnoyers]

* Paul E. McKenney (paulmck@xxxxxxxxxxxxxxxxxx) wrote:
> 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().

I think Peter refers to __this_cpu_add().

Thanks,

Mathieu

> 
> > Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> 
> 
> 							Thanx, Paul
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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