Date:   Mon, 06 Jun 2022 23:16:15 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        Huang Ying <ying.huang@...el.com>,
        Greg Thelen <gthelen@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim C Chen <tim.c.chen@...el.com>,
        Brice Goglin <brice.goglin@...il.com>,
        Michal Hocko <mhocko@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Hesham Almatary <hesham.almatary@...wei.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Alistair Popple <apopple@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Feng Tang <feng.tang@...el.com>,
        Jagdish Gediya <jvgediya@...ux.ibm.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [RFC PATCH v4 2/7] mm/demotion: Expose per node memory tier to
 sysfs

Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com> writes:

> On 6/6/22 9:46 PM, Jonathan Cameron wrote:
>> On Mon, 6 Jun 2022 21:31:16 +0530
>> Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com> wrote:
>> 
>>> On 6/6/22 8:29 PM, Jonathan Cameron wrote:
>>>> On Fri, 3 Jun 2022 14:10:47 +0530
>>>> Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com> wrote:
>>>>    
>>>>> On 5/27/22 7:45 PM, Jonathan Cameron wrote:
>>>>>> On Fri, 27 May 2022 17:55:23 +0530
>>>>>> "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com> wrote:
>>>>>>       
>>>>>>> From: Jagdish Gediya <jvgediya@...ux.ibm.com>
>>>>>>>
>>>>>>> Add support to read/write the memory tierindex for a NUMA node.
>>>>>>>
>>>>>>> /sys/devices/system/node/nodeN/memtier
>>>>>>>
>>>>>>> where N = node id
>>>>>>>
>>>>>>> When read, It list the memory tier that the node belongs to.
>>>>>>>
>>>>>>> When written, the kernel moves the node into the specified
>>>>>>> memory tier, the tier assignment of all other nodes are not
>>>>>>> affected.
>>>>>>>
>>>>>>> If the memory tier does not exist, writing to the above file
>>>>>>> create the tier and assign the NUMA node to that tier.
>>>>>> creates
>>>>>>
>>>>>> There was some discussion in v2 of Wei Xu's RFC that what matter
>>>>>> for creation is the rank, not the tier number.
>>>>>>
>>>>>> My suggestion is move to an explicit creation file such as
>>>>>> memtier/create_tier_from_rank
>>>>>> to which writing the rank gives results in a new tier
>>>>>> with the next device ID and requested rank.
>>>>>
>>>>> I think the below workflow is much simpler.
>>>>>
>>>>> :/sys/devices/system# cat memtier/memtier1/nodelist
>>>>> 1-3
>>>>> :/sys/devices/system# cat node/node1/memtier
>>>>> 1
>>>>> :/sys/devices/system# ls memtier/memtier*
>>>>> nodelist  power  rank  subsystem  uevent
>>>>> /sys/devices/system# ls memtier/
>>>>> default_rank  max_tier  memtier1  power  uevent
>>>>> :/sys/devices/system# echo 2 > node/node1/memtier
>>>>> :/sys/devices/system#
>>>>>
>>>>> :/sys/devices/system# ls memtier/
>>>>> default_rank  max_tier  memtier1  memtier2  power  uevent
>>>>> :/sys/devices/system# cat memtier/memtier1/nodelist
>>>>> 2-3
>>>>> :/sys/devices/system# cat memtier/memtier2/nodelist
>>>>> 1
>>>>> :/sys/devices/system#
>>>>>
>>>>> ie, to create a tier we just write the tier id/tier index to
>>>>> node/nodeN/memtier file. That will create a new memory tier if needed
>>>>> and add the node to that specific memory tier. Since for now we are
>>>>> having 1:1 mapping between tier index to rank value, we can derive the
>>>>> rank value from the memory tier index.
>>>>>
>>>>> For dynamic memory tier support, we can assign a rank value such that
>>>>> new memory tiers are always created such that it comes last in the
>>>>> demotion order.
>>>>
>>>> I'm not keen on having to pass through an intermediate state where
>>>> the rank may well be wrong, but I guess it's not that harmful even
>>>> if it feels wrong ;)
>>>>    
>>>
>>> Any new memory tier added can be of lowest rank (rank - 0) and hence
>>> will appear as the highest memory tier in demotion order.
>> 
>> Depends on driver interaction - if new memory is CXL attached or
>> GPU attached, chances are the driver has an input on which tier
>> it is put in by default.
>> 
>>> User can then
>>> assign the right rank value to the memory tier? Also the actual demotion
>>> target paths are built during memory block online which in most case
>>> would happen after we properly verify that the device got assigned to
>>> the right memory tier with correct rank value?
>> 
>> Agreed, though that may change the model of how memory is brought online
>> somewhat.
>> 
>>>
>>>> Races are potentially a bit of a pain though depending on what we
>>>> expect the usage model to be.
>>>>
>>>> There are patterns (CXL regions for example) of guaranteeing the
>>>> 'right' device is created by doing something like
>>>>
>>>> cat create_tier > temp.txt
>>>> #(temp gets 2 for example on first call then
>>>> # next read of this file gets 3 etc)
>>>>
>>>> cat temp.txt > create_tier
>>>> # will fail if there hasn't been a read of the same value
>>>>
>>>> Assuming all software keeps to the model, then there are no
>>>> race conditions over creation.  Otherwise we have two new
>>>> devices turn up very close to each other and userspace scripting
>>>> tries to create two new tiers - if it races they may end up in
>>>> the same tier when that wasn't the intent.  Then code to set
>>>> the rank also races and we get two potentially very different
>>>> memories in a tier with a randomly selected rank.
>>>>
>>>> Fun and games...  And a fine illustration why sysfs based 'device'
>>>> creation is tricky to get right (and lots of cases in the kernel
>>>> don't).
>>>>    
>>>
>>> I would expect userspace to be careful and verify the memory tier and
>>> rank value before we online the memory blocks backed by the device. Even
>>> if we race, the result would be two device not intended to be part of
>>> the same memory tier appearing at the same tier. But then we won't be
>>> building demotion targets yet. So userspace could verify this, move the
>>> nodes out of the memory tier. Once it is verified, memory blocks can be
>>> onlined.
>> 
>> The race is there and not avoidable as far as I can see. Two processes A and B.
>> 
>> A checks for a spare tier number
>> B checks for a spare tier number
>> A tries to assign node 3 to new tier 2 (new tier created)
>> B tries to assign node 4 to new tier 2 (accidentally hits existing tier - as this
>> is the same method we'd use to put it in the existing tier we can't tell this
>> write was meant to create a new tier).
>> A writes rank 100 to tier 2
>> A checks rank for tier 2 and finds it is 100 as expected.
>> B write rank 200 to tier 2 (it could check if still default but even that is racy)
>> B checks rank for tier 2 rank and finds it is 200 as expected.
>> A onlines memory.
>> B onlines memory.
>> 
>> Both think they got what they wanted, but A definitely didn't.
>> 
>> One work around is the read / write approach and create_tier.
>> 
>> A reads create_tier - gets 2.
>> B reads create_tier - gets 3.
>> A writes 2 to create_tier as that's what it read.
>> B writes 3 to create_tier as that's what it read.
>> 
>> continue with created tiers.  Obviously can exhaust tiers, but if this is
>> root only, could just create lots anyway so no worse off.
>>   
>>>
>>> Having said that can you outline the usage of
>>> memtier/create_tier_from_rank ?
>> 
>> There are corner cases to deal with...
>> 
>> A writes 100 to create_tier_from_rank.
>> A goes looking for matching tier - finds it: tier2
>> B writes 200 to create_tier_from_rank
>> B goes looking for matching tier - finds it: tier3
>> 
>> rest is fine as operating on different tiers.
>> 
>> Trickier is
>> A writes 100 to create_tier_from_rank  - succeed.
>> B writes 100 to create_tier_from_rank  - Could fail, or could just eat it?
>> 
>> Logically this is same as separate create_tier and then a write
>> of rank, but in one operation, but then you need to search
>> for the right one.  As such, perhaps a create_tier
>> that does the read/write pair as above is the best solution.
>> 
>
> This all is good when we allow dynamic rank values. But currently we are 
> restricting ourselves to three rank value as below:
>
> rank   memtier
> 300    memtier0
> 200    memtier1
> 100    memtier2
>
> Now with the above, how do we define a write to create_tier_from_rank. 
> What should be the behavior if user write value other than above defined 
> rank values? Also enforcing the above three rank values as supported 
> implies teaching userspace about them. I am trying to see how to fit
> create_tier_from_rank without requiring the above.
>
> Can we look at implementing create_tier_from_rank when we start 
> supporting dynamic tiers/rank values? ie,
>
> we still allow node/nodeN/memtier. But with dynamic tiers a race free
> way to get a new memory tier would be echo rank > 
> memtier/create_tier_from_rank. We could also say, memtier0/1/2 are 
> kernel defined memory tiers. Writing to memtier/create_tier_from_rank 
> will create new memory tiers above memtier2 with the rank value specified?
>

To keep it compatible we could do this. ie, we just allow creation of
one additional memory tier (memtier3) via the above interface.


:/sys/devices/system/memtier# ls -al
total 0
drwxr-xr-x  4 root root    0 Jun  6 17:39 .
drwxr-xr-x 10 root root    0 Jun  6 17:39 ..
--w-------  1 root root 4096 Jun  6 17:40 create_tier_from_rank
-r--r--r--  1 root root 4096 Jun  6 17:40 default_tier
-r--r--r--  1 root root 4096 Jun  6 17:40 max_tier
drwxr-xr-x  3 root root    0 Jun  6 17:39 memtier1
drwxr-xr-x  2 root root    0 Jun  6 17:40 power
-rw-r--r--  1 root root 4096 Jun  6 17:39 uevent
:/sys/devices/system/memtier# echo 20 > create_tier_from_rank 
:/sys/devices/system/memtier# ls
create_tier_from_rank  default_tier  max_tier  memtier1  memtier3  power  uevent
:/sys/devices/system/memtier# cat memtier3/rank 
20
:/sys/devices/system/memtier# echo 20 > create_tier_from_rank 
bash: echo: write error: No space left on device
:/sys/devices/system/memtier# 

is this good? 

diff --git a/include/linux/memory-tiers.h b/include/linux/memory-tiers.h
index 0468af60d427..a4150120ba24 100644
--- a/include/linux/memory-tiers.h
+++ b/include/linux/memory-tiers.h
@@ -13,7 +13,7 @@
 #define MEMORY_RANK_PMEM	100
 
 #define DEFAULT_MEMORY_TIER	MEMORY_TIER_DRAM
-#define MAX_MEMORY_TIERS  3
+#define MAX_MEMORY_TIERS  4
 
 extern bool numa_demotion_enabled;
 extern nodemask_t promotion_mask;
diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
index c6eb223a219f..7fdee0c4c4ea 100644
--- a/mm/memory-tiers.c
+++ b/mm/memory-tiers.c
@@ -169,7 +169,8 @@ static void insert_memory_tier(struct memory_tier *memtier)
 	list_add_tail(&memtier->list, &memory_tiers);
 }
 
-static struct memory_tier *register_memory_tier(unsigned int tier)
+static struct memory_tier *register_memory_tier(unsigned int tier,
+						unsigned int rank)
 {
 	int error;
 	struct memory_tier *memtier;
@@ -182,7 +183,7 @@ static struct memory_tier *register_memory_tier(unsigned int tier)
 		return NULL;
 
 	memtier->dev.id = tier;
-	memtier->rank = get_rank_from_tier(tier);
+	memtier->rank = rank;
 	memtier->dev.bus = &memory_tier_subsys;
 	memtier->dev.release = memory_tier_device_release;
 	memtier->dev.groups = memory_tier_dev_groups;
@@ -218,9 +219,53 @@ default_tier_show(struct device *dev, struct device_attribute *attr, char *buf)
 }
 static DEVICE_ATTR_RO(default_tier);
 
+
+static struct memory_tier *__get_memory_tier_from_id(int id);
+static ssize_t create_tier_from_rank_store(struct device *dev,
+					   struct device_attribute *attr,
+					   const char *buf, size_t count)
+{
+	int ret, rank;
+	struct memory_tier *memtier;
+
+	ret = kstrtouint(buf, 10, &rank);
+	if (ret)
+		return ret;
+
+	if (ret == MEMORY_RANK_HBM_GPU ||
+	    rank == MEMORY_TIER_DRAM ||
+	    rank == MEMORY_RANK_PMEM)
+		return -EINVAL;
+
+	mutex_lock(&memory_tier_lock);
+	/*
+	 * For now we only support creation of one additional tier via
+	 * this interface.
+	 */
+	memtier = __get_memory_tier_from_id(3);
+	if (!memtier) {
+		memtier = register_memory_tier(3, rank);
+		if (!memtier) {
+			ret = -EINVAL;
+			goto out;
+		}
+	} else {
+		ret = -ENOSPC;
+		goto out;
+	}
+
+	ret = count;
+out:
+	mutex_unlock(&memory_tier_lock);
+	return ret;
+}
+static DEVICE_ATTR_WO(create_tier_from_rank);
+
+
 static struct attribute *memory_tier_attrs[] = {
 	&dev_attr_max_tier.attr,
 	&dev_attr_default_tier.attr,
+	&dev_attr_create_tier_from_rank.attr,
 	NULL
 };
 
@@ -302,7 +347,7 @@ static int __node_set_memory_tier(int node, int tier)
 
 	memtier = __get_memory_tier_from_id(tier);
 	if (!memtier) {
-		memtier = register_memory_tier(tier);
+		memtier = register_memory_tier(tier, get_rank_from_tier(tier));
 		if (!memtier) {
 			ret = -EINVAL;
 			goto out;
@@ -651,7 +696,8 @@ static int __init memory_tier_init(void)
 	 * Register only default memory tier to hide all empty
 	 * memory tier from sysfs.
 	 */
-	memtier = register_memory_tier(DEFAULT_MEMORY_TIER);
+	memtier = register_memory_tier(DEFAULT_MEMORY_TIER,
+				       get_rank_from_tier(DEFAULT_MEMORY_TIER));
 	if (!memtier)
 		panic("%s() failed to register memory tier: %d\n", __func__, ret);
 

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