Messages in this thread | | | Subject | Re: [RFC] Event counters [1/3]: Basic counter functionality | From | Nick Piggin <> | Date | Tue, 03 Jan 2006 16:01:04 +1100 |
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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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