linux-kernel - Re: [RFC 03/13] mm, page_alloc: don't retry initial attempt in slowpath

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Date:	Thu, 12 May 2016 14:48:24 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Johannes Weiner <hannes@...xchg.org>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC 03/13] mm, page_alloc: don't retry initial attempt in
 slowpath

On Tue 10-05-16 09:35:53, Vlastimil Babka wrote:
> After __alloc_pages_slowpath() sets up new alloc_flags and wakes up kswapd, it
> first tries get_page_from_freelist() with the new alloc_flags, as it may
> succeed e.g. due to using min watermark instead of low watermark. This attempt
> does not have to be retried on each loop, since direct reclaim, direct
> compaction and oom call get_page_from_freelist() themselves.
> 
> This patch therefore moves the initial attempt above the retry label. The
> ALLOC_NO_WATERMARKS attempt is kept under retry label as it's special and
> should be retried after each loop.

Yes this makes code both more clear and more logical

> Kswapd wakeups are also done on each retry
> to be safe from potential races resulting in kswapd going to sleep while a
> process (that may not be able to reclaim by itself) is still looping.

I am not sure this is really necessary but it shouldn't be harmful. The
comment clarifies the duplicity so we are not risking "cleanups to
remove duplicated code" I guess.

> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  mm/page_alloc.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 91fbf6f95403..7249949d65ca 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3586,16 +3586,23 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  	 */
>  	alloc_flags = gfp_to_alloc_flags(gfp_mask);
>  
> -retry:
>  	if (gfp_mask & __GFP_KSWAPD_RECLAIM)
>  		wake_all_kswapds(order, ac);
>  
> -	/* This is the last chance, in general, before the goto nopage. */
> +	/*
> +	 * The adjusted alloc_flags might result in immediate success, so try
> +	 * that first
> +	 */
>  	page = get_page_from_freelist(gfp_mask, order,
>  				alloc_flags & ~ALLOC_NO_WATERMARKS, ac);
>  	if (page)
>  		goto got_pg;
>  
> +retry:
> +	/* Ensure kswapd doesn't accidentaly go to sleep as long as we loop */
> +	if (gfp_mask & __GFP_KSWAPD_RECLAIM)
> +		wake_all_kswapds(order, ac);
> +
>  	/* Allocate without watermarks if the context allows */
>  	if (alloc_flags & ALLOC_NO_WATERMARKS) {
>  		/*
> -- 
> 2.8.2
> 
> --
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-- 
Michal Hocko
SUSE Labs