Re: [PATCH v2] mm/page_alloc: Allow high-order pages to be stored on the per-cpu lists

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

On Fri, 11 Jun 2021 14:57:53 +0100 Mel Gorman <mgorman@techsingularity.net> wrote:

> Changelog since v1
> o Fix boot problem on KVM with hotplug memory nodes (ziy)
> o Correct PCP list lookup in bulk page allocator
> 
> The per-cpu page allocator (PCP) only stores order-0 pages. This means
> that all THP and "cheap" high-order allocations including SLUB contends
> on the zone->lock. This patch extends the PCP allocator to store THP and
> "cheap" high-order pages. Note that struct per_cpu_pages increases in
> size to 256 bytes (4 cache lines) on x86-64.
> 
> Note that this is not necessarily a universal performance win because of
> how it is implemented. High-order pages can cause pcp->high to be exceeded
> prematurely for lower-orders so for example, a large number of THP pages
> being freed could release order-0 pages from the PCP lists. Hence, much
> depends on the allocation/free pattern as observed by a single CPU to
> determine if caching helps or hurts a particular workload.
> 
> That said, basic performance testing passed. The following is a netperf
> UDP_STREAM test which hits the relevant patches as some of the network
> allocations are high-order.
> 
> netperf-udp
>                                  5.13.0-rc2             5.13.0-rc2
>                            mm-pcpburst-v3r4   mm-pcphighorder-v1r7
> Hmean     send-64         261.46 (   0.00%)      266.30 *   1.85%*
> Hmean     send-128        516.35 (   0.00%)      536.78 *   3.96%*
> Hmean     send-256       1014.13 (   0.00%)     1034.63 *   2.02%*
> Hmean     send-1024      3907.65 (   0.00%)     4046.11 *   3.54%*
> Hmean     send-2048      7492.93 (   0.00%)     7754.85 *   3.50%*
> Hmean     send-3312     11410.04 (   0.00%)    11772.32 *   3.18%*
> Hmean     send-4096     13521.95 (   0.00%)    13912.34 *   2.89%*
> Hmean     send-8192     21660.50 (   0.00%)    22730.72 *   4.94%*
> Hmean     send-16384    31902.32 (   0.00%)    32637.50 *   2.30%*
> 
> >From a functional point of view, a patch like this is necessary to
> make bulk allocation of high-order pages work with similar performance
> to order-0 bulk allocations. The bulk allocator is not updated in this
> series as it would have to be determined by bulk allocation users how
> they want to track the order of pages allocated with the bulk allocator.
> 
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -198,7 +198,7 @@ extern void post_alloc_hook(struct page *page, unsigned int order,
>  					gfp_t gfp_flags);
>  extern int user_min_free_kbytes;
>  
> -extern void free_unref_page(struct page *page);
> +extern void free_unref_page(struct page *page, unsigned int order);
>  extern void free_unref_page_list(struct list_head *list);
>  
>  extern void zone_pcp_update(struct zone *zone, int cpu_online);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index f24f509c3ee3..8472bae567f0 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -676,10 +676,53 @@ static void bad_page(struct page *page, const char *reason)
>  	add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
>  }
>  
> +static inline unsigned int order_to_pindex(int migratetype, int order)
> +{
> +	int base = order;
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +	if (order > PAGE_ALLOC_COSTLY_ORDER) {
> +		VM_BUG_ON(order != pageblock_order);
> +		base = PAGE_ALLOC_COSTLY_ORDER + 1;
> +	}
> +#else
> +	VM_BUG_ON(order > PAGE_ALLOC_COSTLY_ORDER);
> +#endif
> +
> +	return (MIGRATE_PCPTYPES * base) + migratetype;
> +}
> +
> +static inline int pindex_to_order(unsigned int pindex)
> +{
> +	int order = pindex / MIGRATE_PCPTYPES;
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +	if (order > PAGE_ALLOC_COSTLY_ORDER) {
> +		order = pageblock_order;
> +		VM_BUG_ON(order != pageblock_order);

Somebody has trust issues?

> +	}
> +#else
> +	VM_BUG_ON(order > PAGE_ALLOC_COSTLY_ORDER);
> +#endif
> +
> +	return order;
> +}

Do we really need all these assertions, long-term?