Re: [PATCH 4/9] mm/page_alloc: optimize code layout for __alloc_pages_bulk

Vlastimil Babka
2021-04-12T10:41:48Z

On 3/25/21 12:42 PM, Mel Gorman wrote:
> From: Jesper Dangaard Brouer <brouer@redhat.com>
>
> Looking at perf-report and ASM-code for __alloc_pages_bulk() it is clear
> that the code activated is suboptimal. The compiler guesses wrong and
> places unlikely code at the beginning. Due to the use of WARN_ON_ONCE()
> macro the UD2 asm instruction is added to the code, which confuse the
> I-cache prefetcher in the CPU.

Hm that's weird, WARN_ON_ONCE() uses unlikely() too, so the UD2 should end up in
the out-of-fast-path part?
But anyway.

> [mgorman: Minor changes and rebasing]
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

Acked-By: Vlastimil Babka <vbabka@suse.cz>

> ---
> mm/page_alloc.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index be1e33a4df39..1ec18121268b 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5001,7 +5001,7 @@ int __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> unsigned int alloc_flags;
> int nr_populated = 0;
>
> - if (WARN_ON_ONCE(nr_pages <= 0))
> + if (unlikely(nr_pages <= 0))
> return 0;
>
> /*
> @@ -5048,7 +5048,7 @@ int __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> * If there are no allowed local zones that meets the watermarks then
> * try to allocate a single page and reclaim if necessary.
> */
> - if (!zone)
> + if (unlikely(!zone))
> goto failed;
>
> /* Attempt the batch allocation */
> @@ -5066,7 +5066,7 @@ int __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
>
> page = __rmqueue_pcplist(zone, ac.migratetype, alloc_flags,
> pcp, pcp_list);
> - if (!page) {
> + if (unlikely(!page)) {
> /* Try and get at least one page */
> if (!nr_populated)
> goto failed_irq;
>