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

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

Hi Nick!

On Sat, Dec 31, 2005 at 06:54:25PM +1100, Nick Piggin wrote:
> Marcelo Tosatti wrote:
> 
> >
> >What about this addition to the documentation above, to make it a little 
> >more verbose:
> >
> >	The possible race scenario is restricted to kernel preemption,
> >	and could happen as follows:
> >
> >	thread A				thread B
> >a)	movl    xyz(%ebp), %eax			movl    xyz(%ebp), %eax
> >b)	incl    %eax				incl    %eax
> >c)	movl    %eax, xyz(%ebp)			movl    %eax, xyz(%ebp)
> >
> >Thread A can be preempted in b), and thread B succesfully increments the
> >counter, writing it back to memory. Now thread A resumes execution, with
> >its stale copy of the counter, and overwrites the current counter.
> >
> >Resulting in increments lost.
> >
> >However that should be relatively rare condition.
> >
> 
> 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).

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!

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 343083f..f173e0f 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -83,10 +83,14 @@
 struct page_state {
 	unsigned long nr_dirty;		/* Dirty writeable pages */
 	unsigned long nr_writeback;	/* Pages under writeback */
+					/* also modified from IRQ context */
 	unsigned long nr_unstable;	/* NFS unstable pages */
 	unsigned long nr_page_table_pages;/* Pages used for pagetables */
+					/* only modified from process context */
 	unsigned long nr_mapped;	/* mapped into pagetables */
+					/* only modified from process context */
 	unsigned long nr_slab;		/* In slab */
+					/* also modified from IRQ context */
 #define GET_PAGE_STATE_LAST nr_slab
 
 	/*

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