Re: [PATCH 4/5] page allocator: Pre-emptively wake kswapd when high-order watermarks are hit

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> When a high-order allocation fails, kswapd is kicked so that it reclaims
> at a higher-order to avoid direct reclaimers stall and to help GFP_ATOMIC
> allocations. Something has changed in recent kernels that affect the timing
> where high-order GFP_ATOMIC allocations are now failing with more frequency,
> particularly under pressure.
> 
> This patch pre-emptively checks if watermarks have been hit after a
> high-order allocation completes successfully. If the watermarks have been
> reached, kswapd is woken in the hope it fixes the watermarks before the
> next GFP_ATOMIC allocation fails.
> 
> Warning, this patch is somewhat of a band-aid. If this makes a difference,
> it still implies that something has changed that is either causing more
> GFP_ATOMIC allocations to occur (such as the case with iwlagn wireless
> driver) or make them more likely to fail.

hmm, I'm confused. this description addressed generic high order allocation.
but, 

> 
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> ---
>  mm/page_alloc.c |   33 ++++++++++++++++++++++-----------
>  1 files changed, 22 insertions(+), 11 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 7f2aa3e..851df40 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1596,6 +1596,17 @@ try_next_zone:
>  	return page;
>  }
>  
> +static inline
> +void wake_all_kswapd(unsigned int order, struct zonelist *zonelist,
> +						enum zone_type high_zoneidx)
> +{
> +	struct zoneref *z;
> +	struct zone *zone;
> +
> +	for_each_zone_zonelist(zone, z, zonelist, high_zoneidx)
> +		wakeup_kswapd(zone, order);
> +}
> +
>  static inline int
>  should_alloc_retry(gfp_t gfp_mask, unsigned int order,
>  				unsigned long pages_reclaimed)
> @@ -1730,18 +1741,18 @@ __alloc_pages_high_priority(gfp_t gfp_mask, unsigned int order,

__alloc_pages_high_priority() is only called if ALLOC_NO_WATERMARKS.
ALLOC_NO_WATERMARKS mean PF_MEMALLOC or TIF_MEMDIE and GFP_ATOMIC don't make
nested alloc_pages() (= don't make PF_MEMALLOC case). 
Then, I haven't understand why this patch improve iwlagn GFP_ATOMIC case.

hmm, maybe I missed something. I see the code again tommorow.


>  			congestion_wait(BLK_RW_ASYNC, HZ/50);
>  	} while (!page && (gfp_mask & __GFP_NOFAIL));
>  
> -	return page;
> -}
> -
> -static inline
> -void wake_all_kswapd(unsigned int order, struct zonelist *zonelist,
> -						enum zone_type high_zoneidx)
> -{
> -	struct zoneref *z;
> -	struct zone *zone;
> +	/*
> +	 * If after a high-order allocation we are now below watermarks,
> +	 * pre-emptively kick kswapd rather than having the next allocation
> +	 * fail and have to wake up kswapd, potentially failing GFP_ATOMIC
> +	 * allocations or entering direct reclaim
> +	 */
> +	if (unlikely(order) && page && !zone_watermark_ok(preferred_zone, order,
> +				preferred_zone->watermark[ALLOC_WMARK_LOW],
> +				zone_idx(preferred_zone), ALLOC_WMARK_LOW))
> +		wake_all_kswapd(order, zonelist, high_zoneidx);
>  
> -	for_each_zone_zonelist(zone, z, zonelist, high_zoneidx)
> -		wakeup_kswapd(zone, order);
> +	return page;
>  }
>  
>  static inline int
> -- 
> 1.6.3.3
> 
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