linux-kernel - Re: [EXT] Re: [RFC PATCH 0/2] Node migration between memory tiers

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Date:   Tue, 5 Dec 2023 14:32:20 +0530
From:   Srinivasulu Thanneeru <sthanneeru.opensrc@...ron.com>
To:     Michal Hocko <mhocko@...e.com>
CC:     <aneesh.kumar@...ux.ibm.com>, <linux-cxl@...r.kernel.org>,
        <linux-mm@...ck.org>, <dan.j.williams@...el.com>,
        <hannes@...xchg.org>, <hasanalmaruf@...com>, <haowang3@...com>,
        <ying.huang@...el.com>, <gregory.price@...verge.com>,
        <tj@...nel.org>, <hezhongkun.hzk@...edance.com>, <fvdl@...gle.com>,
        <john@...alactic.com>, <emirakhur@...ron.com>,
        <vtavarespetr@...ron.com>, <Ravis.OpenSrc@...ron.com>,
        <Jonathan.Cameron@...wei.com>, <linux-kernel@...r.kernel.org>,
        <linux-api@...r.kernel.org>
Subject: Re: [EXT] Re: [RFC PATCH 0/2] Node migration between memory tiers



On 12/5/2023 2:21 PM, Michal Hocko wrote:
> CAUTION: EXTERNAL EMAIL. Do not click links or open attachments unless you recognize the sender and were expecting this message.
> 
> 
> On Tue 05-12-23 14:12:17, Srinivasulu Thanneeru wrote:
>>
>>
>> On 12/5/2023 2:05 PM, Michal Hocko wrote:
>>> CAUTION: EXTERNAL EMAIL. Do not click links or open attachments unless you recognize the sender and were expecting this message.
>>>
>>>
>>> On Tue 05-12-23 01:26:07, Srinivasulu Thanneeru wrote:
>>>>
>>>>
>>>> On 12/4/2023 9:13 PM, Michal Hocko wrote:
>>>>> CAUTION: EXTERNAL EMAIL. Do not click links or open attachments unless you recognize the sender and were expecting this message.
>>>>>
>>>>>
>>>>> On Fri 01-12-23 03:34:20, sthanneeru.opensrc@...ron.com wrote:
>>>>>> From: Srinivasulu Thanneeru <sthanneeru.opensrc@...ron.com>
>>>>>>
>>>>>> The memory tiers feature allows nodes with similar memory types
>>>>>> or performance characteristics to be grouped together in a
>>>>>> memory tier. However, there is currently no provision for
>>>>>> moving a node from one tier to another on demand.
>>>>>
>>>>> Could you expand on why this is really needed/necessary? What is the
>>>>> actual usecase?
>>>>
>>>> Hi Michal Hock,
>>>>
>>>> Following two use-cases we have observed.
>>>> 1. It is not accurate to group similar memory types in the same tier,
>>>>      because even similar memory types may have different speed grades.
>>>
>>> Presumably they are grouped based on a HW configuration. Does that mean
>>> that the configuration is wrong? Are you trying to workaround that by
>>> this interface?
>>>
>>>> 2. Some systems boots up with CXL devices and DRAM on the same memory-tier,
>>>> we need a way to move the CXL nodes to the correct tier from the user space.
>>>
>>> Again, could you expand a bit more and explain why this cannot be
>>> configured automatically?
>>
>> Yes, in both cases above, if hardware not automatically populated properly,
>> in that case this interface would help to correct it from user space.
>>
>> We had observed case-2 in our setups.
> 
> How hard it is to address this at the HW level?
> 
> Btw. this is really important piece of context that should be part of
> the changelog. Quite honestly introducing user interfaces solely to
> workaround HW issues seems a rather weak justification. Are there any
> usecases you can think of where this would be useful?

I'm not sure how difficult to fix it in the hardware.

Sure, i will capture the use-cases in the change log, will be sending V2 
by changing interface from adistance_offset to memtier_overwrite to 
avoid complicated math for finding offset at user-level.

Thank you Michal Hocko for the feedback.

> --
> Michal Hocko
> SUSE Labs