Re: [PATCH 00/10] rcu: Cleanup RCU tree initialization

[Alexander Gordeev]

On Mon, Mar 09, 2015 at 02:35:42PM -0700, Paul E. McKenney wrote:
> On Mon, Mar 09, 2015 at 09:36:52AM +0000, Alexander Gordeev wrote:
> > On Mon, Mar 09, 2015 at 09:34:04AM +0100, Alexander Gordeev wrote:
> > > Hi Paul,
> > > 
> > > Here is cleanup of RCU tree initialization rebased on linux-rcu rcu/next
> > > repo, as you requested. Please, note an extra patch #10 that was not
> > > present in the first post.
> > 
> > Paul,
> > 
> > Please, ignore patch #10 for now. I missed to notice rcu_node::grpnum is
> > used in tracing, so the patch is incomplete. I am not sure why trailing
> > spaces in seq_printf(m, "%lx/%lx->%lx %c%c>%c %d:%d ^%d    ", ....) are
> > needed for, so not sure if "^%d" part should be removed (possibly with
> > the traling spaces) or replaced with three spaces.
> 
> OK, dropping this one for the moment.
> 
> The original use of ->grpnum was for manual debugging purposes.  Yes, you
> can get the same information out of ->grpmask, but the number is easier
> to read.  And on the debugfs trace information, ->grpnum is printed,
> but ->grpmask is not.
> 
> The trailing spaces on the seq_printf() allow the rcu_node data to be
> printed on a single line, while still allowing the eye to pick out
> where one rcu_node structure's data ends and the next one begins.
> 
> So here are the choices, as far as I can see:
> 
> 1.	Leave ->grpnum as is.
> 
> 2.	Remove ->grpnum, but regenerate it in print_one_rcu_state(),
> 	for example, by counting the number of rcu_node structures
> 	since the last ->level change.
> 
> 3.	Drop ->grpnum and also remove it from the debugfs tracing.
> 	The reader can rely on the ->grplo and ->grphi fields to
> 	work out where this rcu_node structure fits in, but we
> 	lose the visual indication of any bugs in computing these
> 	quantities.
> 
> 4.	Drop ->grpnum and replace it with ->grpmask.  This seems a
> 	bit obtuse to me.
> 
> 5.	Redesign print_one_rcu_state()'s output from scratch.
> 
> #1 has certain advantages from a laziness viewpoint.  #2 would open up
> some space in the rcu_node structure, but space really isn't an issue
> for that structure given that huge systems have only 257 of them and
> the really small systems use Tiny RCU instead.  #3 might be OK, but I
> am not really convinced.  #4 seems a bit ugly.  I am not signing up
> for #5, in part because not all that many people use RCU's debugfs
> output, so I don't see the point in investing the time.
> 
> But what did you have in mind?

I probably should have marked this patch as an RFC. Given your summary
#1 seems as the best choice.

However, I have something else in mind, indeed. What is the reason to
have 'grpnum' and 'level' as u8 while, say 'grplo' and 'grphi' - as int?
IOW, do we conserve on memory for this structure or not?

Thanks!

> 							Thanx, Paul
> 

-- 
Regards,
Alexander Gordeev
agordeev@xxxxxxxxxx
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