linux-kernel - Re: [PATCH v5 4/9] mm/demotion: Build demotion targets based on explicit memory tiers

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Date:   Wed, 8 Jun 2022 13:49:16 +0530
From:   Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
To:     Ying Huang <ying.huang@...el.com>, linux-mm@...ck.org,
        akpm@...ux-foundation.org
Cc:     Wei Xu <weixugc@...gle.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>,
        Jonathan Cameron <Jonathan.Cameron@...wei.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: [PATCH v5 4/9] mm/demotion: Build demotion targets based on
 explicit memory tiers

On 6/8/22 12:20 PM, Ying Huang wrote:
> On Fri, 2022-06-03 at 19:12 +0530, Aneesh Kumar K.V wrote:
>> This patch switch the demotion target building logic to use memory tiers
>> instead of NUMA distance. All N_MEMORY NUMA nodes will be placed in the
>> default tier 1 and additional memory tiers will be added by drivers like
>> dax kmem.
>>
>> This patch builds the demotion target for a NUMA node by looking at all
>> memory tiers below the tier to which the NUMA node belongs. The closest node
>> in the immediately following memory tier is used as a demotion target.
>>
>> Since we are now only building demotion target for N_MEMORY NUMA nodes
>> the CPU hotplug calls are removed in this patch.
>>
>> The rank approach allows us to keep memory tier device IDs stable even if there
>> is a need to change the tier ordering among different memory tiers. e.g. DRAM
>> nodes with CPUs will always be on memtier1, no matter how many tiers are higher
>> or lower than these nodes. A new memory tier can be inserted into the tier
>> hierarchy for a new set of nodes without affecting the node assignment of any
>> existing memtier, provided that there is enough gap in the rank values for the
>> new memtier.
>>
>> The absolute value of "rank" of a memtier doesn't necessarily carry any meaning.
>> Its value relative to other memtiers decides the level of this memtier in the tier
>> hierarchy.
>>
>> For now, This patch supports hardcoded rank values which are 300, 200, & 100 for
>> memory tiers 0,1 & 2 respectively.
>>
>> Below is the sysfs interface to read the rank values of memory tier,
>> /sys/devices/system/memtier/memtierN/rank
>>
>> This interface is read only for now. Write support can be added when there is
>> a need of flexibility of more number of memory tiers(> 3) with flexibile ordering
>> requirement among them.
>>
>> Suggested-by: Wei Xu <weixugc@...gle.com>
>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>
>> ---
>>   include/linux/memory-tiers.h |   5 +
>>   include/linux/migrate.h      |  13 --
>>   mm/memory-tiers.c            | 269 ++++++++++++++++++++++++
>>   mm/migrate.c                 | 394 -----------------------------------
>>   mm/vmstat.c                  |   4 -
>>   5 files changed, 274 insertions(+), 411 deletions(-)
> 
> It appears that you moved some code from migrate.c to memory-tiers.c and
> change them.  If so, please separate the change.  That is, one patch
> only move the code, the other change the code.  This will make it easier
> to find out what is changed.

That was how it was done in earlier version. That is we did change 
establish_migration within the same file. The changes we are doing here 
was so different that it was mentioned that it gets very hard to review
in a context diff. Hence this patch where we killed the old code and did 
the new code in memory-tiers.c. I could still move the code to 
memory-tiers.c and do the changes on top of that. Infact I do have a 
patch that does similar code movement in the series. But the diff was 
not useful for an easy review.

-aneesh