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

Nick Piggin
2006-01-03T05:01:04Z

On Sat, 2005-12-31 at 18:26 -0200, Marcelo Tosatti wrote:
> Hi Nick!
>

Hey Marcelo!

> On Sat, Dec 31, 2005 at 06:54:25PM +1100, Nick Piggin wrote:
> >
> > Hi Guys,
> >
> > I've been waiting for some mm/ patches to clear from -mm before commenting
> > too much... however I see that this patch is actually against -mm itself,
> > with my __mod_page_state stuff in it... that makes the page state accounting
> > much lighter weight AND is not racy.
>
> It is racy with reference to preempt (please refer to the race condition
> described above):
>
> diff -puN mm/rmap.c~mm-page_state-opt mm/rmap.c
> --- devel/mm/rmap.c~mm-page_state-opt 2005-12-13 22:25:01.000000000 -0800
> +++ devel-akpm/mm/rmap.c 2005-12-13 22:25:01.000000000 -0800
> @@ -451,7 +451,11 @@ static void __page_set_anon_rmap(struct
>
> page->index = linear_page_index(vma, address);
>
> - inc_page_state(nr_mapped);
> + /*
> + * nr_mapped state can be updated without turning off
> + * interrupts because it is not modified via interrupt.
> + */
> + __inc_page_state(nr_mapped);
> }
>
> And since "nr_mapped" is not a counter for debugging purposes only, you
> can't be lazy with reference to its consistency.
>
> I would argue that you need a preempt save version for this important
> counters, surrounded by preempt_disable/preempt_enable (which vanish
> if one selects !CONFIG_PREEMPT).
>

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.

There were some runtime warnings that come up when this patch first
went into -mm because of a silly typo, however that should now be
resolved too.

> As Christoph notes, debugging counter consistency can be lazy, not even
> requiring correct preempt locking (hum, this is debatable, needs careful
> verification).
>
> > So I'm not exactly sure why such a patch as this is wanted now? Are there
> > any more xxx_page_state hotspots? (I admit to only looking at page faults,
> > page allocator, and page reclaim).
>
> A consolidation of the good parts of both would be interesting.
>
> I don't see much point in Christoph's naming change to "event_counter",
> why are you doing that?
>
> And follows an addition to your's mm-page_state-opt-docs.patch. Still
> need to verify "nr_dirty" and "nr_unstable".
>
> Happy new year!
>

Thanks, happy new year to you too!

--
SUSE Labs, Novell Inc.

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