linux-kernel - Re: [PATCH v3 0/7] mm: demotion: Introduce new node state N_DEMOTION

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Date:   Wed, 27 Apr 2022 08:27:19 +0530
From:   Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
To:     "ying.huang@...el.com" <ying.huang@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Jagdish Gediya <jvgediya@...ux.ibm.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, baolin.wang@...ux.alibaba.com,
        dave.hansen@...ux.intel.com, shy828301@...il.com,
        weixugc@...gle.com, gthelen@...gle.com, dan.j.williams@...el.com
Subject: Re: [PATCH v3 0/7] mm: demotion: Introduce new node state
 N_DEMOTION_TARGETS

On 4/27/22 6:59 AM, ying.huang@...el.com wrote:
> On Mon, 2022-04-25 at 20:14 +0530, Aneesh Kumar K V wrote:
>> On 4/25/22 7:27 PM, Jonathan Cameron wrote:
>>> On Mon, 25 Apr 2022 16:45:38 +0530
>>> Jagdish Gediya <jvgediya@...ux.ibm.com> wrote:
>>>
>>>> On Sun, Apr 24, 2022 at 11:19:53AM +0800, ying.huang@...el.com wrote:
>>>>> On Sat, 2022-04-23 at 01:25 +0530, Jagdish Gediya wrote:
>>>>>> Some systems(e.g. PowerVM) can have both DRAM(fast memory) only
>>>>>> NUMA node which are N_MEMORY and slow memory(persistent memory)
>>>>>> only NUMA node which are also N_MEMORY. As the current demotion
>>>>>> target finding algorithm works based on N_MEMORY and best distance,
>>>>>> it will choose DRAM only NUMA node as demotion target instead of
>>>>>> persistent memory node on such systems. If DRAM only NUMA node is
>>>>>> filled with demoted pages then at some point new allocations can
>>>>>> start falling to persistent memory, so basically cold pages are in
>>>>>> fast memor (due to demotion) and new pages are in slow memory, this
>>>>>> is why persistent memory nodes should be utilized for demotion and
>>>>>> dram node should be avoided for demotion so that they can be used
>>>>>> for new allocations.
>>>>>>
>>>>>> Current implementation can work fine on the system where the memory
>>>>>> only numa nodes are possible only for persistent/slow memory but it
>>>>>> is not suitable for the like of systems mentioned above.
>>>>>
>>>>> Can you share the NUMA topology information of your machine?  And the
>>>>> demotion order before and after your change?
>>>>>
>>>>> Whether it's good to use the PMEM nodes as the demotion targets of the
>>>>> DRAM-only node too?
>>>>
>>>> $ numactl -H
>>>> available: 2 nodes (0-1)
>>>> node 0 cpus: 0 1 2 3 4 5 6 7
>>>> node 0 size: 14272 MB
>>>> node 0 free: 13392 MB
>>>> node 1 cpus:
>>>> node 1 size: 2028 MB
>>>> node 1 free: 1971 MB
>>>> node distances:
>>>> node   0   1
>>>>     0:  10  40
>>>>     1:  40  10
>>>>
>>>> 1) without N_DEMOTION_TARGETS patch series, 1 is demotion target
>>>>      for 0 even when 1 is DRAM node and there is no demotion targets for 1.
>>>
>>> I'm not convinced the distinction between DRAM and persistent memory is
>>> valid. There will definitely be systems with a large pool
>>> of remote DRAM (and potentially no NV memory) where the right choice
>>> is to demote to that DRAM pool.
>>>
>>> Basing the decision on whether the memory is from kmem or
>>> normal DRAM doesn't provide sufficient information to make the decision.
>>>
>>
>> Hence the suggestion for the ability to override this from userspace.
>> Now, for example, we could build a system with memory from the remote
>> machine (memory inception in case of power which will mostly be plugged
>> in as regular hotpluggable memory ) and a slow CXL memory or OpenCAPI
>> memory.
>>
>> In the former case, we won't consider that for demotion with this series
>> because that is not instantiated via dax kmem. So yes definitely we
>> would need the ability to override this from userspace so that we could
>> put these remote memory NUMA nodes as demotion targets if we want.
>>>>
> 
> Is there a driver for the device (memory from the remote machine)?  If
> so, we can adjust demotion order for it in the driver.
> 

At this point, it is managed by hypervisor, is hotplugged into the the 
LPAR with more additional properties specified via device tree. So there 
is no inception specific device driver.

> In general, I think that we can adjust demotion order inside kernel from
> various information sources.  In addition to ACPI SLIT, we also have
> HMAT, kmem driver, other drivers, etc.
> 

Managing inception memory will any way requires a userspace component to 
track the owner machine for the remote memory. So we should be ok to 
have userspace manage demotion order.

-aneesh