Re: [PATCH v5 14/14] mm, compaction: try to capture the just-created high-order freepage

[Joonsoo Kim]

On Tue, Jul 29, 2014 at 05:34:37PM +0200, Vlastimil Babka wrote:
> >>@@ -570,6 +572,14 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> >>  	unsigned long flags;
> >>  	bool locked = false;
> >>  	struct page *page = NULL, *valid_page = NULL;
> >>+	unsigned long capture_pfn = 0;   /* current candidate for capturing */
> >>+	unsigned long next_capture_pfn = 0; /* next candidate for capturing */
> >>+
> >>+	if (cc->order > 0 && cc->order <= pageblock_order && capture) {
> >>+		/* This may be outside the zone, but we check that later */
> >>+		capture_pfn = low_pfn & ~((1UL << cc->order) - 1);
> >>+		next_capture_pfn = ALIGN(low_pfn + 1, (1UL << cc->order));
> >>+	}
> >
> >Instead of inserting capture logic to common code (compaction and
> >CMA), could you add it only to compaction code such as
> >isolate_migratepages(). Capture logic needs too many hooks as you see
> >on below snippets. And it makes code so much complicated.
> 
> Could do it in isolate_migratepages() for whole pageblocks only (as
> David's patch did), but that restricts the usefulness. Or maybe do
> it fine grained by calling isolate_migratepages_block() multiple
> times. But the overhead of multiple calls would probably suck even
> more for lower-order compactions. For CMA the added overhead is
> basically only checks for next_capture_pfn that will be always
> false, so predictable. And mostly just in branches where isolation
> is failing, which is not the CMA's "fast path" I guess?

You can do it find grained with compact_control's migratepages list
or new private list. If some pages are isolated and added to this list,
you can check pfn of page on this list and determine appropriate capture
candidate page. This approach can give us more flexibility for
choosing capture candidate without adding more complexity to
common function. For example, you can choose capture candidate if
there are XX isolated pages in certain range.

> 
> But I see you're talking about "complicated", not overhead. Well
> it's 4 hunks inside the isolate_migratepages_block() for loop. I
> don't think it's *that* bad, thanks to how the function was cleaned
> up by the previosu patches.
> Hmm but you made me realize I could make it nicer by doing a "goto
> isolation_fail" which would handle the next_capture_pfn update at a
> single place.
> 
> >>+static bool compact_capture_page(struct compact_control *cc)
> >>+{
> >>+	struct page *page = *cc->capture_page;
> >>+	int cpu;
> >>+
> >>+	if (!page)
> >>+		return false;
> >>+
> >>+	/* Unsafe check if it's worth to try acquiring the zone->lock at all */
> >>+	if (PageBuddy(page) && page_order_unsafe(page) >= cc->order)
> >>+		goto try_capture;
> >>+
> >>+	/*
> >>+	 * There's a good chance that we have just put free pages on this CPU's
> >>+	 * lru cache and pcplists after the page migrations. Drain them to
> >>+	 * allow merging.
> >>+	 */
> >>+	cpu = get_cpu();
> >>+	lru_add_drain_cpu(cpu);
> >>+	drain_local_pages(NULL);
> >>+	put_cpu();
> >
> >Just for curiosity.
> >
> >If lru_add_drain_cpu() is cheap enough to capture high order page, why
> >__alloc_pages_direct_compact() doesn't call it before
> >get_page_from_freelist()?
> 
> No idea. I guess it wasn't noticed at the time that page migration
> uses putback_lru_page() on the page that was freed, which puts it
> into the lru_add cache, only to be freed. I think it would be better
> to free the page immediately in this case, and use lru_add cache
> only for pages that will really go to lru.
> 
> Heck, it could be even better to tell page migration to skip
> pcplists as well, to avoid drain_local_pages. Often you migrate
> because you want to use the original page for something. NUMA
> balancing migrations are different, I guess.
> 
> >>--- a/mm/page_alloc.c
> >>+++ b/mm/page_alloc.c
> >>@@ -1489,9 +1489,11 @@ static int __isolate_free_page(struct page *page, unsigned int order)
> >>  {
> >>  	unsigned long watermark;
> >>  	struct zone *zone;
> >>+	struct free_area *area;
> >>  	int mt;
> >>+	unsigned int freepage_order = page_order(page);
> >>
> >>-	BUG_ON(!PageBuddy(page));
> >>+	VM_BUG_ON_PAGE((!PageBuddy(page) || freepage_order < order), page);
> >>
> >>  	zone = page_zone(page);
> >>  	mt = get_pageblock_migratetype(page);
> >>@@ -1506,9 +1508,12 @@ static int __isolate_free_page(struct page *page, unsigned int order)
> >>  	}
> >>
> >
> >In __isolate_free_page(), we check zone_watermark_ok() with order 0.
> >But normal allocation logic would check zone_watermark_ok() with requested
> >order. Your capture logic uses __isolate_free_page() and it would
> >affect compaction success rate significantly. And it means that
> >capture logic allocates high order page on page allocator
> >too aggressively compared to other component such as normal high order
> 
> It's either that, or the extra lru drain that makes the different.
> But the "aggressiveness" would in fact mean better accuracy.
> Watermark checking may be inaccurate. Especially when memory is
> close to the watermark and there is only a single high-order page
> that would satisfy the allocation.

If this "aggressiveness" means better accuracy, fixing general
function, watermark_ok() is better than adding capture logic.

But, I guess that there is a reason that watermark_ok() is so
conservative. If page allocator aggressively provides high order page,
future atomic high order page request cannot succeed easily. For
preventing this situation, watermark_ok() should be conservative.

Thanks.
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