Re: kswapd0: excessive CPU usage

[Jiri Slaby <jslaby@suse.cz>]

On 10/12/2012 03:57 PM, Mel Gorman wrote:
> mm: vmscan: scale number of pages reclaimed by reclaim/compaction only in direct reclaim
> 
> Jiri Slaby reported the following:
> 
> 	(It's an effective revert of "mm: vmscan: scale number of pages
> 	reclaimed by reclaim/compaction based on failures".)
> 	Given kswapd had hours of runtime in ps/top output yesterday in the
> 	morning and after the revert it's now 2 minutes in sum for the last 24h,
> 	I would say, it's gone.
> 
> The intention of the patch in question was to compensate for the loss of
> lumpy reclaim. Part of the reason lumpy reclaim worked is because it
> aggressively reclaimed pages and this patch was meant to be a
> sane compromise.
> 
> When compaction fails, it gets deferred and both compaction and
> reclaim/compaction is deferred avoid excessive reclaim. However, since
> commit c6543459 (mm: remove __GFP_NO_KSWAPD), kswapd is woken up each time
> and continues reclaiming which was not taken into account when the patch
> was developed.
> 
> As it is not taking deferred compaction into account in this path it scans
> aggressively before falling out and making the compaction_deferred check in
> compaction_ready. This patch avoids kswapd scaling pages for reclaim and
> leaves the aggressive reclaim to the process attempting the THP
> allocation.
> 
> Signed-off-by: Mel Gorman <mgorman@suse.de>
> ---
>  mm/vmscan.c |   10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 2624edc..2b7edfa 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1763,14 +1763,20 @@ static bool in_reclaim_compaction(struct scan_control *sc)
>  #ifdef CONFIG_COMPACTION
>  /*
>   * If compaction is deferred for sc->order then scale the number of pages
> - * reclaimed based on the number of consecutive allocation failures
> + * reclaimed based on the number of consecutive allocation failures. This
> + * scaling only happens for direct reclaim as it is about to attempt
> + * compaction. If compaction fails, future allocations will be deferred
> + * and reclaim avoided. On the other hand, kswapd does not take compaction
> + * deferral into account so if it scaled, it could scan excessively even
> + * though allocations are temporarily not being attempted.
>   */
>  static unsigned long scale_for_compaction(unsigned long pages_for_compaction,
>  			struct lruvec *lruvec, struct scan_control *sc)
>  {
>  	struct zone *zone = lruvec_zone(lruvec);
>  
> -	if (zone->compact_order_failed <= sc->order)
> +	if (zone->compact_order_failed <= sc->order &&
> +	    !current_is_kswapd())
>  		pages_for_compaction <<= zone->compact_defer_shift;
>  	return pages_for_compaction;
>  }

Yes, applying this instead of the revert fixes the issue as well.

thanks,
-- 
js
suse labs

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