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Message-ID: <d953e533-e175-17ba-c065-dfd1354a858f@linux.alibaba.com>
Date: Sun, 14 Nov 2021 22:40:35 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Yang Shi <shy828301@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Huang Ying <ying.huang@...el.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Zi Yan <ziy@...dia.com>, Oscar Salvador <osalvador@...e.de>,
zhongjiang-ali@...ux.alibaba.com,
Xunlei Pang <xlpang@...ux.alibaba.com>,
Linux MM <linux-mm@...ck.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] mm: migrate: Support multiple target nodes demotion
On 2021/11/13 3:05, Yang Shi wrote:
> On Thu, Nov 11, 2021 at 6:28 PM Baolin Wang
> <baolin.wang@...ux.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@...ux.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.
[1]
https://lore.kernel.org/all/c02bcbc04faa7a2c852534e9cd58a91c44494657.1636016609.git.baolin.wang@linux.alibaba.com/
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