Re: [PATCH] mm: migrate: Support multiple target nodes demotion

["Huang, Ying" <ying.huang@intel.com>]

Baolin Wang <baolin.wang@linux.alibaba.com> writes:

> We have some machines with multiple memory types like below, which
> have one fast (DRAM) memory node and two slow (persistent memory) memory
> nodes. According to current node demotion, if node 0 fills up,
                              ~~~~~~~~~~~~~

node demotion policy?


> its memory should be migrated to node 1, when node 1 fills up, its
> memory will be migrated to node 2: node 0 -> node 1 -> node 2 ->stop.
>
> But this is not efficient and suitbale memory migration route
> for our machine with multiple slow memory nodes. Since the distance
> between node 0 to node 1 and node 0 to node 2 is equal, and memory
> migration between slow memory nodes will increase persistent memory
> bandwidth greatly, which will hurt the whole system's performance.
>
> Thus for this case, we can treat the slow memory node 1 and node 2
> as a whole slow memory region, and we should migrate memory from
> node 0 to node 1 and node 2 if node 0 fills up.
>
> This patch changes the node_demotion data structure to support multiple
> target nodes, and establishes the migration path to support multiple
> target nodes with validating if the node distance is the best or not.
>
> available: 3 nodes (0-2)
> node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
> node 0 size: 62153 MB
> node 0 free: 55135 MB
> node 1 cpus:
> node 1 size: 127007 MB
> node 1 free: 126930 MB
> node 2 cpus:
> node 2 size: 126968 MB
> node 2 free: 126878 MB
> node distances:
> node   0   1   2
>   0:  10  20  20
>   1:  20  10  20
>   2:  20  20  10
>
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
> Changes from RFC v2:
>  - Change to 'short' type for target nodes array.
>  - Remove nodemask instead selecting target node directly.
>  - Add WARN_ONCE() if the target nodes exceed the maximum value.
>
> Changes from RFC v1:
>  - Re-define the node_demotion structure.
>  - Set up multiple target nodes by validating the node distance.
>  - Add more comments.
> ---
>  mm/migrate.c | 138 +++++++++++++++++++++++++++++++++++++++++++----------------
>  1 file changed, 102 insertions(+), 36 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index cf25b00..7f1d745 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -50,6 +50,7 @@
>  #include <linux/ptrace.h>
>  #include <linux/oom.h>
>  #include <linux/memory.h>
> +#include <linux/random.h>
>  
>  #include <asm/tlbflush.h>
>  
> @@ -1119,12 +1120,25 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
>   *
>   * This is represented in the node_demotion[] like this:
>   *
> - *	{  1, // Node 0 migrates to 1
> - *	   2, // Node 1 migrates to 2
> - *	  -1, // Node 2 does not migrate
> - *	   4, // Node 3 migrates to 4
> - *	   5, // Node 4 migrates to 5
> - *	  -1} // Node 5 does not migrate
> + *	{  nr=1, nodes[0]=1 }, // Node 0 migrates to 1
> + *	{  nr=1, nodes[0]=2 }, // Node 1 migrates to 2
> + *	{  nr=0, nodes[0]=-1 }, // Node 2 does not migrate
> + *	{  nr=1, nodes[0]=4 }, // Node 3 migrates to 4
> + *	{  nr=1, nodes[0]=5 }, // Node 4 migrates to 5
> + *	{  nr=0, nodes[0]=-1} // Node 5 does not migrate
> + *
> + * Moreover some systems may have multiple same class memory
> + * types.  Suppose a system has one socket with 3 memory nodes,

s/same class memory types/slow memory nodes/

?

We don't support multiple fast memory types, right?

> + * node 0 is fast memory type, and node 1/2 both are slow memory
> + * type, and the distance between fast memory node and slow
> + * memory node is same. So the migration path should be:
> + *
> + *	0 -> 1/2 -> stop
> + *
> + * This is represented in the node_demotion[] like this:
> + *	{ nr=2, {nodes[0]=1, nodes[1]=2} }, // Node 0 migrates to node 1 and node 2
> + *	{ nr=0, nodes[0]=-1, }, // Node 1 dose not migrate
> + *	{ nr=0, nodes[0]=-1, }, // Node 2 does not migrate
>   */
>  
>  /*
> @@ -1135,8 +1149,13 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
>   * must be held over all reads to ensure that no cycles are
>   * observed.
>   */
> -static int node_demotion[MAX_NUMNODES] __read_mostly =
> -	{[0 ...  MAX_NUMNODES - 1] = NUMA_NO_NODE};
> +#define DEMOTION_TARGET_NODES 15
> +struct demotion_nodes {
> +	unsigned short nr;
> +	short nodes[DEMOTION_TARGET_NODES];
> +};
> +
> +static struct demotion_nodes node_demotion[MAX_NUMNODES] __read_mostly;

If MAX_NUMNODES is 1024, the total size will be (16 * 2 * 1024) = 32K
bytes.  That appears too large.  We may consider to allocate
node_demotion[] dynamically.

Best Regards,
Huang, Ying