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

[Peter Zijlstra]

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 ;-)

> @@ -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.

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