Re: [PATCH -mm v8 6/7] proc: add kpageidle file

On Wed, Jul 15, 2015 at 12:42:28PM -0700, Andres Lagar-Cavilla wrote:

On Wed, Jul 15, 2015 at 6:54 AM, Vladimir Davydov
[off-list ref] wrote:

[...]

quoted

+static void kpageidle_clear_pte_refs(struct page *page)
+{
+       struct rmap_walk_control rwc = {
+               .rmap_one = kpageidle_clear_pte_refs_one,
+               .anon_lock = page_lock_anon_vma_read,
+       };
+       bool need_lock;
+
+       if (!page_mapped(page) ||

Question: what about mlocked pages? Is there any point in calculating
their idleness?

Those can be filtered out with the aid of /proc/kpageflags (this is what
the script attached to patch #0 of the series actually does). We have to
read the latter anyway in order to get information about THP. That said,
I prefer not to introduce any artificial checks for locked memory. Who
knows, may be one day somebody will use this API to track access pattern
to an mlocked area.

quoted

+           !page_rmapping(page))

Not sure, does this skip SwapCache pages? Is there any point in
calculating their idleness?

A SwapCache page may be mapped, and if it is we should not skip it. If
it is unmapped, we have nothing to do.

Regarding idleness of SwapCache pages, I think we shouldn't
differentiate them from other user pages here, because a shmem/anon page
can migrate to-and-fro the swap cache occasionally during a
memory-active workload, and we don't want to lose its idle status then.

quoted

+               return;
+
+       need_lock = !PageAnon(page) || PageKsm(page);
+       if (need_lock && !trylock_page(page))
+               return;
+
+       rmap_walk(page, &rwc);
+
+       if (need_lock)
+               unlock_page(page);
+}

[...]

quoted

@@ -1754,6 +1754,11 @@ static void __split_huge_page_refcount(struct page *page,
                /* clear PageTail before overwriting first_page */
                smp_wmb();

+               if (page_is_young(page))
+                       set_page_young(page_tail);
+               if (page_is_idle(page))
+                       set_page_idle(page_tail);
+

Why not in the block above?

page_tail->flags |= (page->flags &
...
#ifdef CONFIG_WHATEVER_IT_WAS
1 << PG_idle
1 << PG_young
#endif

Too many ifdef's :-/ Note, the flags can be in page_ext, which mean we
would have to add something like this

#if defined(CONFIG_WHATEVER_IT_WAS) && defined(CONFIG_64BIT)
1 << PG_idle
1 << PG_young
#endif
<...>
#ifndef CONFIG_64BIT
if (page_is_young(page))
	set_page_young(page_tail);
if (page_is_idle(page))
	set_page_idle(page_tail);
#endif

which IMO looks less readable than what we have now.

Thanks,
Vladimir

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