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

Marcelo Tosatti
2005-12-31T20:26:02Z

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

/*