Re: [PATCH 2/2] mm: vmscan: If kswapd has been running too long, allow it to sleep

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

On Mon, 16 May 2011 16:06:57 +0100
Mel Gorman <mgorman@suse.de> wrote:

> Under constant allocation pressure, kswapd can be in the situation where
> sleeping_prematurely() will always return true even if kswapd has been
> running a long time. Check if kswapd needs to be scheduled.
> 
> Signed-off-by: Mel Gorman <mgorman@suse.de>
> Acked-by: Rik van Riel <riel@redhat.com>
> ---
>  mm/vmscan.c |    4 ++++
>  1 files changed, 4 insertions(+), 0 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index af24d1e..4d24828 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2251,6 +2251,10 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
>  	unsigned long balanced = 0;
>  	bool all_zones_ok = true;
>  
> +	/* If kswapd has been running too long, just sleep */
> +	if (need_resched())
> +		return false;
> +
>  	/* If a direct reclaimer woke kswapd within HZ/10, it's premature */
>  	if (remaining)
>  		return true;

I'm a bit worried by this one.

Do we really fully understand why kswapd is continuously running like
this?  The changelog makes me think "no" ;)

Given that the page-allocating process is madly reclaiming pages in
direct reclaim (yes?) and that kswapd is madly reclaiming pages on a
different CPU, we should pretty promptly get into a situation where
kswapd can suspend itself.  But that obviously isn't happening.  So
what *is* going on?

Secondly, taking an up-to-100ms sleep in response to a need_resched()
seems pretty savage and I suspect it risks undesirable side-effects.  A
plain old cond_resched() would be more cautious.  But presumably
kswapd() is already running cond_resched() pretty frequently, so why
didn't that work?

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