Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU from both process and interrupt context

[Paolo Bonzini]

On 06/06/2017 12:53, Peter Zijlstra wrote:
> On Mon, Jun 05, 2017 at 03:09:50PM -0700, Paul E. McKenney wrote:
>> There would be a slowdown if 1) fast this_cpu_inc is not available and
>> cannot be implemented (this usually means that atomic_inc has implicit
>> memory barriers),
> 
> I don't get this.
> 
> How is per-cpu crud related to being strongly ordered?
> 
> this_cpu_ has 3 forms:
> 
> 	x86:		single instruction
> 	arm64,s390:	preempt_disable()+atomic_op
> 	generic:	local_irq_save()+normal_op
> 
> Only s390 is TSO, arm64 is very much a weak arch.

Right, and thus arm64 can implement a fast this_cpu_inc using LL/SC.
s390 cannot because its atomic_inc has implicit memory barriers.

s390's this_cpu_inc is *faster* than the generic one, but still pretty slow.

>>                   and 2) local_irq_save/restore is slower than disabling
>> preemption.  The main architecture with these constraints is s390, which
>> however is already paying the price in __srcu_read_unlock and has not
>> complained.
> 
> IIRC only PPC (and hopefully soon x86) has a local_irq_save() that is as
> fast as preempt_disable().

1 = arch-specific this_cpu_inc is available
2 = local_irq_save/restore as fast as preempt_disable/enable

If either 1 or 2 are true, this patch makes SRCU faster or equal

x86 (single instruction): 1 = true, 2 = false   -> ok
arm64 (weakly ordered):   1 = true, 2 = false   -> ok
powerpc:                  1 = false, 2 = true   -> ok
s390:                     1 = false, 2 = false  -> slower

For other LL/SC architectures, notably arm, fast this_cpu_* ops not yet
available, but could be written pretty easily.

>> A valid optimization on s390 would be to skip the smp_mb;
>> AIUI, this_cpu_inc implies a memory barrier (!) due to its implementation.
> 
> You mean the s390 this_cpu_inc() in specific, right? Because
> this_cpu_inc() in general does not imply any such thing.

Yes, of course, this is only for s390.

Alternatively, we could change the counters to atomic_t and use
smp_mb__{before,after}_atomic, as in the (unnecessary) srcutiny patch.
That should shave a few cycles on x86 too, since "lock inc" is faster
than "inc; mfence".  For srcuclassic (and stable) however I'd rather
keep the simple __this_cpu_inc -> this_cpu_inc change.

Paolo