Re: [PATCH v4 3/3] mm/free_pcppages_bulk: prefetch buddy while not holding lock

[Andrew Morton <akpm@linux-foundation.org>]

On Thu,  1 Mar 2018 14:28:45 +0800 Aaron Lu <aaron.lu@intel.com> wrote:

> When a page is freed back to the global pool, its buddy will be checked
> to see if it's possible to do a merge. This requires accessing buddy's
> page structure and that access could take a long time if it's cache cold.
> 
> This patch adds a prefetch to the to-be-freed page's buddy outside of
> zone->lock in hope of accessing buddy's page structure later under
> zone->lock will be faster. Since we *always* do buddy merging and check
> an order-0 page's buddy to try to merge it when it goes into the main
> allocator, the cacheline will always come in, i.e. the prefetched data
> will never be unused.
> 
> In the meantime, there are two concerns:
> 1 the prefetch could potentially evict existing cachelines, especially
>   for L1D cache since it is not huge;
> 2 there is some additional instruction overhead, namely calculating
>   buddy pfn twice.
> 
> For 1, it's hard to say, this microbenchmark though shows good result but
> the actual benefit of this patch will be workload/CPU dependant;
> For 2, since the calculation is a XOR on two local variables, it's expected
> in many cases that cycles spent will be offset by reduced memory latency
> later. This is especially true for NUMA machines where multiple CPUs are
> contending on zone->lock and the most time consuming part under zone->lock
> is the wait of 'struct page' cacheline of the to-be-freed pages and their
> buddies.
> 
> Test with will-it-scale/page_fault1 full load:
> 
> kernel      Broadwell(2S)  Skylake(2S)   Broadwell(4S)  Skylake(4S)
> v4.16-rc2+  9034215        7971818       13667135       15677465
> patch2/3    9536374 +5.6%  8314710 +4.3% 14070408 +3.0% 16675866 +6.4%
> this patch 10338868 +8.4%  8544477 +2.8% 14839808 +5.5% 17155464 +2.9%
> Note: this patch's performance improvement percent is against patch2/3.
> 
> ...
>
> @@ -1150,6 +1153,18 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  				continue;
>  
>  			list_add_tail(&page->lru, &head);
> +
> +			/*
> +			 * We are going to put the page back to the global
> +			 * pool, prefetch its buddy to speed up later access
> +			 * under zone->lock. It is believed the overhead of
> +			 * calculating buddy_pfn here can be offset by reduced
> +			 * memory latency later.
> +			 */
> +			pfn = page_to_pfn(page);
> +			buddy_pfn = __find_buddy_pfn(pfn, 0);
> +			buddy = page + (buddy_pfn - pfn);
> +			prefetch(buddy);

What is the typical list length here?  Maybe it's approximately the pcp
batch size which is typically 128 pages?

If so, I'm a bit surprised that it is effective to prefetch 128 page
frames before using any them for real.  I guess they'll fit in the L2
cache.   Thoughts?