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Message-ID: <e3a88e5f-0e72-1f4e-ca6c-4158dc25eb2e@linux.alibaba.com>
Date: Wed, 10 Nov 2021 09:30:11 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: akpm@...ux-foundation.org, dave.hansen@...ux.intel.com,
ziy@...dia.com, osalvador@...e.de, shy828301@...il.com,
zhongjiang-ali@...ux.alibaba.com, xlpang@...ux.alibaba.com,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2] mm: migrate: Support multiple target nodes
demotion
On 2021/11/10 8:47, Huang, Ying writes:
> Baolin Wang <baolin.wang@...ux.alibaba.com> writes:
>
>> On 2021/11/9 15:53, Huang, Ying writes:
>>> Baolin Wang <baolin.wang@...ux.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,
>>>> 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@...ux.alibaba.com>
>>>> ---
>>>> 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 | 132 +++++++++++++++++++++++++++++++++++++++++++----------------
>>>> 1 file changed, 96 insertions(+), 36 deletions(-)
>>>>
>>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>>> index cf25b00..95f170d 100644
>>>> --- a/mm/migrate.c
>>>> +++ b/mm/migrate.c
>>>> @@ -1119,12 +1119,25 @@ 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,
>>>> + * 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 +1148,13 @@ 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;
>>>> + int nodes[DEMOTION_TARGET_NODES];
>>> Why we cannot use "unsigned short" for nodes[]?
>>
>> I think the default value of target node should be NUMA_NO_NODE(-1),
>> so a signed type is more suitable. I can change to 'short' type.
>
> Yes. 'short' is better.
>
>>>
>>>> +};
>>>> +
>>>> +static struct demotion_nodes node_demotion[MAX_NUMNODES] __read_mostly;
>>>> /**
>>>> * next_demotion_node() - Get the next node in the demotion path
>>>> @@ -1149,7 +1167,9 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
>>>> */
>>>> int next_demotion_node(int node)
>>>> {
>>>> - int target;
>>>> + struct demotion_nodes *current_node_demotion = &node_demotion[node];
>>>> + int target, i;
>>>> + nodemask_t target_nodes = NODE_MASK_NONE;
>>>> /*
>>>> * node_demotion[] is updated without excluding this
>>>> @@ -1161,9 +1181,21 @@ int next_demotion_node(int node)
>>>> * node_demotion[] reads need to be consistent.
>>>> */
>>>> rcu_read_lock();
>>>> - target = READ_ONCE(node_demotion[node]);
>>>> + for (i = 0; i < DEMOTION_TARGET_NODES; i++) {
>>>> + target = READ_ONCE(current_node_demotion->nodes[i]);
>>>> + if (target == NUMA_NO_NODE)
>>>> + break;
>>>> +
>>>> + node_set(target, target_nodes);
>>> Why do we need a nodemask? Why not just find a target node from
>>> current_node_demotion->nodes[] randomly and directly?
>>
>> I think nodemask is scalable in future if we want to add more
>> requirements to select the target node if necessary. Anyway now I have
>> no strong preference with the nodemask, and can change to select the
>> target node randomly and directly, which are something like below.
>>
>> + target_nr = READ_ONCE(current_node_demotion->nr);
>> +
>> + if (target_nr == 0) {
>> + target = NUMA_NO_NODE;
>> + goto out;
>> + } else if (target_nr == 1) {
>> + index = 0;
>> + } else {
>> + /*
>> + * If there are multiple target nodes, just select one
>> + * target node randomly.
>> + */
>> + index = get_random_int() % target_nr;
>> + }
>> +
>> + target = READ_ONCE(current_node_demotion->nodes[index]);
>
> This looks generally OK. You may consider about memory order.
Yes. Sorry, I just cut out a piece of sample code. They are must under
RCU lock, and using READ_ONCE() to avoid compiler reordering or read
merging like the comments said.
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