Re: [RFC] Event counters [1/3]: Basic counter functionality

[Marcelo Tosatti <marcelo.tosatti@cyclades.com>]

On Tue, Jan 03, 2006 at 04:11:46PM +1100, Nick Piggin wrote:
> On Mon, 2006-01-02 at 19:40 -0200, Marcelo Tosatti wrote:
> 
> > Nick, 
> > 
> > The following patch:
> > 
> > - Moves the lightweight "inc/dec" versions of mod_page_state variants
> > to three underscores, making those the default for locations where enough
> > locks are held.
> > 
> 
> I guess I was hoping to try to keep it simple, and just have two
> variants, the __ version would require the caller to do the locking.

I see - one point is that the two/three underscore versions make
it clear that preempt is required, though, but it might be a bit
over-complicated as you say.

Well, its up to you - please rearrange the patch as you wish and merge
up?

> In cases like eg. allocstall, they should happen infrequently enough
> that the extra complexity is probably not worth worrying about.

True, but it reduces kernel code, which is always good.

> I don't think I commented about the preempt race though (and requirement
> to have preempt off from process context), which obviously can be a
> problem as you say (though I think things are currently safe?).

"I think it should not be racy because the function should always be
called with the page table lock held, which disables preempt. I guess
the comment should be explicit about that as well."

Yes, you're right! My bad.

> > - Make the two-underscore version disable and enable preemption, which 
> > is required to avoid preempt-related races which can result in missed
> > updates.
> > 
> > - Extends the lightweight version usage in page reclaim, 
> > pte allocation, and a few other codepaths.
> > 
> 
> I guess nr_dirty looks OK in the places it can be put under tree_lock.
> 
> nr_page_table_pages is OK because ptl should be held to prevent preempt.
> 
> pgrotated and pgactivate should be good because of lru_lock.
> 
> Thanks for going through these!

There's still probably a few more counters but the ones covered till now 
should be the most significant ones performance-wise.