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 <rient...@google.com>
> ---
> 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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