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Date:   Thu, 7 Oct 2021 16:35:54 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Mel Gorman <mgorman@techsingularity.net>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] mm: Optimise put_pages_list()
Message-Id: <20211007163554.d9088a65f0e293e2bd906a56@linux-foundation.org>
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        <20211007123109.6a49c7c625e414acf7546c89@linux-foundation.org>
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On Thu, 7 Oct 2021 21:55:21 +0100 Matthew Wilcox <willy@infradead.org> wrote:

> On Thu, Oct 07, 2021 at 12:31:09PM -0700, Andrew Morton wrote:
> > On Thu,  7 Oct 2021 20:21:37 +0100 "Matthew Wilcox (Oracle)" <willy@infradead.org> wrote:
> > 
> > > Instead of calling put_page() one page at a time, pop pages off
> > > the list if their refcount was too high and pass the remainder to
> > > put_unref_page_list().  This should be a speed improvement, but I have
> > > no measurements to support that.  Current callers do not care about
> > > performance, but I hope to add some which do.
> > 
> > Don't you think it would actually be slower to take an additional pass
> > across the list?  If the list is long enough to cause cache thrashing. 
> > Maybe it's faster for small lists.
> 
> My first response is an appeal to authority -- release_pages() does
> this same thing.  Only it takes an array, constructs a list and passes
> that to put_unref_page_list().  So if that's slower (and lists _are_
> slower than arrays), we should have a put_unref_page_array().

And put_unref_page_list() does two passes across the list!

<quietly sobs>

Here is my beautiful release_pages(), as disrtibuted in linux-2.5.33:

void release_pages(struct page **pages, int nr)
{
	int i;
	struct pagevec pages_to_free;
	struct zone *zone = NULL;

	pagevec_init(&pages_to_free);
	for (i = 0; i < nr; i++) {
		struct page *page = pages[i];
		struct zone *pagezone;

		if (PageReserved(page) || !put_page_testzero(page))
			continue;

		pagezone = page_zone(page);
		if (pagezone != zone) {
			if (zone)
				spin_unlock_irq(&zone->lru_lock);
			zone = pagezone;
			spin_lock_irq(&zone->lru_lock);
		}
		if (TestClearPageLRU(page))
			del_page_from_lru(zone, page);
		if (page_count(page) == 0) {
			if (!pagevec_add(&pages_to_free, page)) {
				spin_unlock_irq(&zone->lru_lock);
				pagevec_free(&pages_to_free);
				pagevec_init(&pages_to_free);
				spin_lock_irq(&zone->lru_lock);
			}
		}
	}
	if (zone)
		spin_unlock_irq(&zone->lru_lock);

	pagevec_free(&pages_to_free);
}

I guess the current version is some commentary on the aging process?


> Second, we can follow through the code paths and reason about it.
> 
> Before:
> 
> while (!list_empty(pages)) {
> 	put_page(victim);
> 		page = compound_head(page);
> 		if (put_page_testzero(page))
> 			__put_page(page);
> 				__put_single_page(page)
> 					__page_cache_release(page);
> 					mem_cgroup_uncharge(page);
> 					<---
> free_unref_page(page, 0);
> 	free_unref_page_prepare()
>         local_lock_irqsave(&pagesets.lock, flags);
>         free_unref_page_commit(page, pfn, migratetype, order);
>         local_unlock_irqrestore(&pagesets.lock, flags);
> 
> After:
> 
> free_unref_page_list(pages);
>         list_for_each_entry_safe(page, next, list, lru) {
>                 if (!free_unref_page_prepare(page, pfn, 0)) {
>         }
> 
>         local_lock_irqsave(&pagesets.lock, flags);
>         list_for_each_entry_safe(page, next, list, lru) {
> 		free_unref_page_commit()
> 	}
>         local_unlock_irqrestore(&pagesets.lock, flags);
> 
> So the major win here is that we disable/enable interrupts once per
> batch rather than once per page.

Perhaps that's faster if the list is fully cached.

Any feelings for how often release_pages() will be passed a huge enough
list for this to occur?