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Date: Tue, 16 Nov 2021 10:58:18 +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 2021/11/16 3:06, Yang Shi wrote:
> On Sun, Nov 14, 2021 at 6:40 AM Baolin Wang
> <baolin.wang@...ux.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@...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.
>
> 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.
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