linux-kernel - Re: [PATCH V4] mm: Add sysfs interface to dump each node's zonelist information

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Message-Id: <57D63C01.9090309@linux.vnet.ibm.com>
Date:   Mon, 12 Sep 2016 10:54:17 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>
CC:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH V4] mm: Add sysfs interface to dump each node's zonelist
 information

On 09/09/2016 07:06 PM, Michal Hocko wrote:
> On Thu 08-09-16 08:16:58, Anshuman Khandual wrote:
>> > Each individual node in the system has a ZONELIST_FALLBACK zonelist
>> > and a ZONELIST_NOFALLBACK zonelist. These zonelists decide fallback
>> > order of zones during memory allocations. Sometimes it helps to dump
>> > these zonelists to see the priority order of various zones in them.
>> > 
>> > Particularly platforms which support memory hotplug into previously
>> > non existing zones (at boot), this interface helps in visualizing
>> > which all zonelists of the system at what priority level, the new
>> > hot added memory ends up in. POWER is such a platform where all the
>> > memory detected during boot time remains with ZONE_DMA for good but
>> > then hot plug process can actually get new memory into ZONE_MOVABLE.
>> > So having a way to get the snapshot of the zonelists on the system
>> > after memory or node hot[un]plug is desirable. This change adds one
>> > new sysfs interface (/sys/devices/system/memory/system_zone_details)
>> > which will fetch and dump this information.
> I am still not sure I understand why this is helpful and who is the
> consumer for this interface and how it will benefit from the
> information. Dave (who doesn't seem to be on the CC list re-added) had
> another objection that this breaks one-value-per-file rule for sysfs
> files.

It helps in understanding the relative priority of each memory zone of the
system during various allocation scenarios. Its particularly helpful after
hotplug/unplug of additional memory into previously non existing zone on
a node.

> 
> This all smells like a debugging feature to me and so it should go into
> debugfs.

Sure, will make it a debugfs interface.