Re: [patch v3] mm, thp: only collapse hugepages to nodes with affinity for zone_reclaim_mode

[Dave Hansen]

On 07/16/2014 05:59 PM, David Rientjes wrote:
> Commit 9f1b868a13ac ("mm: thp: khugepaged: add policy for finding target 
> node") improved the previous khugepaged logic which allocated a 
> transparent hugepages from the node of the first page being collapsed.
> 
> However, it is still possible to collapse pages to remote memory which may 
> suffer from additional access latency.  With the current policy, it is 
> possible that 255 pages (with PAGE_SHIFT == 12) will be collapsed remotely 
> if the majority are allocated from that node.
> 
> When zone_reclaim_mode is enabled, it means the VM should make every attempt
> to allocate locally to prevent NUMA performance degradation.  In this case,
> we do not want to collapse hugepages to remote nodes that would suffer from
> increased access latency.  Thus, when zone_reclaim_mode is enabled, only
> allow collapsing to nodes with RECLAIM_DISTANCE or less.
> 
> There is no functional change for systems that disable zone_reclaim_mode.
> 
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
> ---
>  v2: only change behavior for zone_reclaim_mode per Dave Hansen
>  v3: optimization based on previous node counts per Vlastimil Babka
> 
>  mm/huge_memory.c | 31 +++++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2234,6 +2234,30 @@ static void khugepaged_alloc_sleep(void)
>  static int khugepaged_node_load[MAX_NUMNODES];
>  
>  #ifdef CONFIG_NUMA
> +static bool khugepaged_scan_abort(int nid)
> +{
> +	int i;
> +
> +	/*
> +	 * If zone_reclaim_mode is disabled, then no extra effort is made to
> +	 * allocate memory locally.
> +	 */
> +	if (!zone_reclaim_mode)
> +		return false;
> +
> +	/* If there is a count for this node already, it must be acceptable */
> +	if (khugepaged_node_load[nid])
> +		return false;
> +
> +	for (i = 0; i < MAX_NUMNODES; i++) {
> +		if (!khugepaged_node_load[i])
> +			continue;
> +		if (node_distance(nid, i) > RECLAIM_DISTANCE)
> +			return true;
> +	}
> +	return false;
> +}
> +
>  static int khugepaged_find_target_node(void)
>  {
>  	static int last_khugepaged_target_node = NUMA_NO_NODE;
> @@ -2309,6 +2333,11 @@ static struct page
>  	return *hpage;
>  }
>  #else
> +static bool khugepaged_scan_abort(int nid)
> +{
> +	return false;
> +}

Minor nit: I guess this makes it more explicit, but this #ifdef is
unnecessary in practice because we define zone_reclaim_mode this way:

#ifdef CONFIG_NUMA
extern int zone_reclaim_mode;
#else
#define zone_reclaim_mode 0
#endif

Looks fine to me otherwise, though.  Definitely addresses the concerns I
had about RECLAIM_DISTANCE being consulted directly.
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