linux-kernel - Re: [PATCH] mm: migrate: set demotion targets differently

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Message-ID: <87ilruy5zt.fsf@linux.ibm.com>
Date:   Thu, 31 Mar 2022 12:15:58 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Jagdish Gediya <jvgediya@...ux.ibm.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        baolin.wang@...ux.alibaba.com, dave.hansen@...ux.intel.com
Subject: Re: [PATCH] mm: migrate: set demotion targets differently

"Huang, Ying" <ying.huang@...el.com> writes:

> "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com> writes:
>
>> "Huang, Ying" <ying.huang@...el.com> writes:
>>
>>> Hi, Jagdish,
>>>
>>> Jagdish Gediya <jvgediya@...ux.ibm.com> writes:
>>>
>>
>> ...
>>
>>>> e.g. with below NUMA topology, where node 0 & 1 are
>>>> cpu + dram nodes, node 2 & 3 are equally slower memory
>>>> only nodes, and node 4 is slowest memory only node,
>>>>
>>>> available: 5 nodes (0-4)
>>>> node 0 cpus: 0 1
>>>> node 0 size: n MB
>>>> node 0 free: n MB
>>>> node 1 cpus: 2 3
>>>> node 1 size: n MB
>>>> node 1 free: n MB
>>>> node 2 cpus:
>>>> node 2 size: n MB
>>>> node 2 free: n MB
>>>> node 3 cpus:
>>>> node 3 size: n MB
>>>> node 3 free: n MB
>>>> node 4 cpus:
>>>> node 4 size: n MB
>>>> node 4 free: n MB
>>>> node distances:
>>>> node   0   1   2   3   4
>>>>   0:  10  20  40  40  80
>>>>   1:  20  10  40  40  80
>>>>   2:  40  40  10  40  80
>>>>   3:  40  40  40  10  80
>>>>   4:  80  80  80  80  10
>>>>
>>>> The existing implementation gives below demotion targets,
>>>>
>>>> node    demotion_target
>>>>  0              3, 2
>>>>  1              4
>>>>  2              X
>>>>  3              X
>>>>  4		X
>>>>
>>>> With this patch applied, below are the demotion targets,
>>>>
>>>> node    demotion_target
>>>>  0              3, 2
>>>>  1              3, 2
>>>>  2              3
>>>>  3              4
>>>>  4		X
>>>
>>> For such machine, I think the perfect demotion order is,
>>>
>>> node    demotion_target
>>>  0              2, 3
>>>  1              2, 3
>>>  2              4
>>>  3              4
>>>  4              X
>>
>> I guess the "equally slow nodes" is a confusing definition here. Now if the
>> system consists of 2 1GB equally slow memory and the firmware doesn't want to
>> differentiate between them, firmware can present a single NUMA node
>> with 2GB capacity? The fact that we are finding two NUMA nodes is a hint
>> that there is some difference between these two memory devices. This is
>> also captured by the fact that the distance between 2 and 3 is 40 and not 10.
>
> Do you have more information about this?

Not sure I follow the question there. I was checking shouldn't firmware
do a single NUMA node if two memory devices are of the same type? How will
optane present such a config? Both the DIMMs will have the same
proximity domain value and hence dax kmem will add them to the same NUMA
node?

If you are suggesting that firmware doesn't do that, then I agree with you
that a demotion target like the below is good. 

 node    demotion_target
  0              2, 3
  1              2, 3
  2              4
  3              4
  4              X

We can also achieve that with a smiple change as below.

@@ -3120,7 +3120,7 @@ static void __set_migration_target_nodes(void)
 {
 	nodemask_t next_pass	= NODE_MASK_NONE;
 	nodemask_t this_pass	= NODE_MASK_NONE;
-	nodemask_t used_targets = NODE_MASK_NONE;
+	nodemask_t this_pass_used_targets = NODE_MASK_NONE;
 	int node, best_distance;
 
 	/*
@@ -3141,17 +3141,20 @@ static void __set_migration_target_nodes(void)
 	/*
 	 * To avoid cycles in the migration "graph", ensure
 	 * that migration sources are not future targets by
-	 * setting them in 'used_targets'.  Do this only
+	 * setting them in 'this_pass_used_targets'.  Do this only
 	 * once per pass so that multiple source nodes can
 	 * share a target node.
 	 *
-	 * 'used_targets' will become unavailable in future
+	 * 'this_pass_used_targets' will become unavailable in future
 	 * passes.  This limits some opportunities for
 	 * multiple source nodes to share a destination.
 	 */
-	nodes_or(used_targets, used_targets, this_pass);
+	nodes_or(this_pass_used_targets, this_pass_used_targets, this_pass);
 
 	for_each_node_mask(node, this_pass) {
+
+		nodemask_t used_targets = this_pass_used_targets;
+
 		best_distance = -1;
 
 		/*