Re: [PATCH v3] mm: migrate: Support multiple target nodes demotion

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2021/11/16 3:06, Yang Shi wrote:
> On Sun, Nov 14, 2021 at 6:40 AM Baolin Wang
> <baolin.wang@linux.alibaba.com> wrote:
>>
>>
>>
>> On 2021/11/13 3:05, Yang Shi wrote:
>>> On Thu, Nov 11, 2021 at 6:28 PM Baolin Wang
>>> <baolin.wang@linux.alibaba.com> wrote:
>>>>
>>>> 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 policy, if node 0 fills up,
>>>> 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 v2:
>>>>    - Redefine the DEMOTION_TARGET_NODES macro according to the
>>>>      MAX_NUMNODES.
>>>>    - Change node_demotion to a pointer and allocate it dynamically.
>>>>
>>>> Changes from v1:
>>>>    - Add a new patch to allocate the node_demotion dynamically.
>>>>    - Update some comments.
>>>>    - Simplify some variables' name.
>>>>
>>>> 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 | 167 ++++++++++++++++++++++++++++++++++++++++++++++-------------
>>>>    1 file changed, 132 insertions(+), 35 deletions(-)
>>>>
>>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>>> index cf25b00..9b8a813 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 slow memory nodes.
>>>> + * Suppose a system has one socket with 3 memory nodes, 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 +1149,20 @@ 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 DEFAULT_DEMOTION_TARGET_NODES 15
>>>> +
>>>> +#if MAX_NUMNODES < DEFAULT_DEMOTION_TARGET_NODES
>>>> +#define DEMOTION_TARGET_NODES  (MAX_NUMNODES - 1)
>>>> +#else
>>>> +#define DEMOTION_TARGET_NODES  DEFAULT_DEMOTION_TARGET_NODES
>>>> +#endif
>>>> +
>>>> +struct demotion_nodes {
>>>> +       unsigned short nr;
>>>> +       short nodes[DEMOTION_TARGET_NODES];
>>>> +};
>>>> +
>>>> +static struct demotion_nodes *node_demotion __read_mostly;
>>>>
>>>>    /**
>>>>     * next_demotion_node() - Get the next node in the demotion path
>>>> @@ -1149,8 +1175,15 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
>>>>     */
>>>>    int next_demotion_node(int node)
>>>>    {
>>>> +       struct demotion_nodes *nd;
>>>> +       unsigned short target_nr, index;
>>>>           int target;
>>>>
>>>> +       if (!node_demotion)
>>>> +               return NUMA_NO_NODE;
>>>> +
>>>> +       nd = &node_demotion[node];
>>>> +
>>>>           /*
>>>>            * node_demotion[] is updated without excluding this
>>>>            * function from running.  RCU doesn't provide any
>>>> @@ -1161,9 +1194,28 @@ int next_demotion_node(int node)
>>>>            * node_demotion[] reads need to be consistent.
>>>>            */
>>>>           rcu_read_lock();
>>>> -       target = READ_ONCE(node_demotion[node]);
>>>> -       rcu_read_unlock();
>>>> +       target_nr = READ_ONCE(nd->nr);
>>>> +
>>>> +       switch (target_nr) {
>>>> +       case 0:
>>>> +               target = NUMA_NO_NODE;
>>>> +               goto out;
>>>> +       case 1:
>>>> +               index = 0;
>>>> +               break;
>>>> +       default:
>>>> +               /*
>>>> +                * If there are multiple target nodes, just select one
>>>> +                * target node randomly.
>>>> +                */
>>>> +               index = get_random_int() % target_nr;
>>>
>>> Sorry for chiming in late. I don't get why not select demotion targe > node interleave? TBH, it makes more sense to me. Random is ok, but at
>>> least I'd expect to see some explanation about why random is used.
>>
>> My first version patch[1] already did round-robin to select target node.
>> For interleave (or round-robin), we should introduce another member to
>> record last selected target node, as Dave and Ying said, that will cause
>> cache ping-pong to hurt performance, or introduce per-cpu data to avoid
>> this, which seems more complicated now.
> 
> Thanks. It should be better to have some words in the commit log or
> code to elaborate this? Someone else may have the same question in the
> future.

OK. I saw Andrew has queued v4 patch, and I can create an incremental 
patch to add some comments to explain this. Thanks.