linux-kernel - Re: [PATCH] mm: call cond_resched() per MAX_ORDER_NR

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Date:	Mon, 18 Nov 2013 11:02:32 -0800
From:	Dave Hansen <dave@...1.net>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
CC:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	dave.jiang@...el.com, akpm@...ux-foundation.org, dhillf@...il.com,
	Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH] mm: call cond_resched() per MAX_ORDER_NR_PAGES pages
 copy

On 11/18/2013 10:54 AM, Naoya Horiguchi wrote:
> diff --git a/mm/migrate.c b/mm/migrate.c
> index cb5d152b58bc..661ff5f66591 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -454,7 +454,8 @@ static void __copy_gigantic_page(struct page *dst, struct page *src,
>  	struct page *src_base = src;
>  
>  	for (i = 0; i < nr_pages; ) {
> -		cond_resched();
> +		if (i % MAX_ORDER_NR_PAGES == 0)
> +			cond_resched();
>  		copy_highpage(dst, src);

This is certainly OK on x86, but remember that MAX_ORDER can be
overridden by a config variable.  Just picking one at random:

config FORCE_MAX_ZONEORDER
        int "Maximum zone order"
        range 9 64 if PPC64 && PPC_64K_PAGES
...

Would it be OK to only resched once every 2^63 pages? ;)

Really, though, a lot of things seem to have MAX_ORDER set up so that
it's at 256MB or 512MB.  That's an awful lot to do between rescheds.
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