Re: [RFC 0/5] rcu: Introduce leaf_node_for_each_mask_possible_cpu() and its friend

[Paul E. McKenney]

On Sat, Dec 10, 2016 at 09:36:29PM +0800, Boqun Feng wrote:
> On Fri, Dec 09, 2016 at 08:28:05PM -0800, Paul E. McKenney wrote:
> > On Sat, Dec 10, 2016 at 08:45:38AM +0800, Boqun Feng wrote:
> > > On Fri, Dec 09, 2016 at 03:49:45PM -0800, Paul E. McKenney wrote:
> > > > On Fri, Dec 09, 2016 at 04:48:22PM +0800, Boqun Feng wrote:
> > > > > Hi Paul,
> > > > > 
> > > > > While reading the discussion at:
> > > > > 
> > > > > https://marc.info/?l=linux-kernel&m=148044253400769
> > > > 
> > > > This discussion was for stalls specifically, rather than for routine
> > > > scans of the bitmasks.
> > > > 
> > > > But it does look to save some code, so worth looking into.
> > > > 
> > > > > I figured we might use this fact to save some extra checks in RCU core code,
> > > > > currently we iterate over all the possible CPUs on a leaf node, check whether
> > > > > they were masked in a certain mask and do something. However, given the fact
> > > > > that the masks on a leaf node should always be sparse than the corresponding
> > > > > part of cpu_possible_mask, we'd better iterate over all bits in a mask and
> > > > > check whether the corresponding CPU is possible or not.
> > > > > 
> > > > > So I made this RFC, I did a simple build/boot/rcutorture test on my box with
> > > > > SMP=4, nothing bad happens. Currently I'm waiting for the 0day and trying to
> > > > > test this one a bigger system, in the meanwhile, looking forwards to any
> > > > > comment and suggestion.
> > > > > 
> > > > > So thoughts?
> > > > 
> > > > By analogy with for_each_cpu() and for_each_possible_cpu(), the name
> > > > should instead be for_each_leaf_node_cpu(), the tradition of excessively
> > > > long names in RCU notwithstanding.  ;-)
> > > > 
> > > 
> > > Make sense ;-)
> > > 
> > > I think it's more appropriate to call it for_each_leaf_node_mask_cpu(),
> > > because we don't iterate all cpus of a leaf node. The word "possible"
> > > could be dropped because obviously we won't iterate over "impossible"
> > > cpus in a leaf node ;-)
> > 
> > C'mon, Boqun!  The for_each_leaf_node_cpu() is not only consistent
> > with the for_each_cpu() family, it is shorter!  ;-)
> 
> Sure ;-) But for_each_leaf_node_cpu() seems like an operation that
> iterates over _all_ cpus in a leaf node, but I actually implement it as
> an operation that iterates only the _masked_ cpus. So I feel like word
> "mask" better be added in the name.

Although that is a fair point, the same can be said of for_each_cpu().
Which people seem to be able to use without undue pain.

> If we call it for_each_leaf_node_cpu(rnp, mask,...), we will rely on the
> hope that readers could figure it out what the primitive actually does
> by the indication of the parameter @mask.
> 
> I like shorter names too, but not sure whether putting "mask" in the
> name is better. After all, naming is one of the most difficult
> challenges in programming ;-)

The two most difficult challenges in programming are the last two hard
things that the person speaking worked on.  ;-)

Consistency is more important than the stand-alone understanding of
this particular name.  You can always add a comment pointing out that
it follows for_each_cpu().

							Thanx, Paul

> Regards,
> Boqun
> 
> > 							Thanx, Paul
> > 
> > > Will modify that in next version.
> > > 
> > > Regards,
> > > Boqun
> > > 
> > > > 							Thanx, Paul
> > > > 
> > 
> >