linux-kernel - Re: [PATCH 1/4] mm: vmscan: Correct check for kswapd sleeping in sleeping

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Date:	Tue, 28 Jun 2011 14:49:00 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Pádraig Brady <P@...igBrady.com>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Colin King <colin.king@...onical.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Andrew Lutomirski <luto@....edu>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] mm: vmscan: Correct check for kswapd sleeping in
 sleeping_prematurely

On Fri, 24 Jun 2011 15:44:54 +0100
Mel Gorman <mgorman@...e.de> wrote:

> During allocator-intensive workloads, kswapd will be woken frequently
> causing free memory to oscillate between the high and min watermark.
> This is expected behaviour.
> 
> A problem occurs if the highest zone is small.  balance_pgdat()
> only considers unreclaimable zones when priority is DEF_PRIORITY
> but sleeping_prematurely considers all zones. It's possible for this
> sequence to occur
> 
>   1. kswapd wakes up and enters balance_pgdat()
>   2. At DEF_PRIORITY, marks highest zone unreclaimable
>   3. At DEF_PRIORITY-1, ignores highest zone setting end_zone
>   4. At DEF_PRIORITY-1, calls shrink_slab freeing memory from
>         highest zone, clearing all_unreclaimable. Highest zone
>         is still unbalanced
>   5. kswapd returns and calls sleeping_prematurely
>   6. sleeping_prematurely looks at *all* zones, not just the ones
>      being considered by balance_pgdat. The highest small zone
>      has all_unreclaimable cleared but but the zone is not
>      balanced. all_zones_ok is false so kswapd stays awake
> 
> This patch corrects the behaviour of sleeping_prematurely to check
> the zones balance_pgdat() checked.

But kswapd is making progress: it's reclaiming slab.  Eventually that
won't work any more and all_unreclaimable will not be cleared and the
condition will fix itself up?



btw,

	if (!sleeping_prematurely(...))
		sleep();

hurts my brain.  My brain would prefer

	if (kswapd_should_sleep(...))
		sleep();

no?

> Reported-and-tested-by: Pádraig Brady <P@...igBrady.com>

But what were the before-and-after observations?  I don't understand
how this can cause a permanent cpuchew by kswapd.

> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2323,7 +2323,7 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
>  		return true;
>  
>  	/* Check the watermark levels */
> -	for (i = 0; i < pgdat->nr_zones; i++) {
> +	for (i = 0; i <= classzone_idx; i++) {
>  		struct zone *zone = pgdat->node_zones + i;
>  
>  		if (!populated_zone(zone))

The patch looks sensible.
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