linux-kernel - Re: [RFC PATCH] Add /proc/<pid>/numa

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Date:   Wed, 2 May 2018 15:28:59 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Prakash Sangappa <prakash.sangappa@...cle.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-api@...r.kernel.org, mhocko@...e.com,
        kirill.shutemov@...ux.intel.com, n-horiguchi@...jp.nec.com,
        drepper@...il.com, rientjes@...gle.com,
        Naoya Horiguchi <nao.horiguchi@...il.com>
Subject: Re: [RFC PATCH] Add /proc/<pid>/numa_vamaps for numa node information

On 05/02/2018 02:33 PM, Andrew Morton wrote:
> On Tue,  1 May 2018 22:58:06 -0700 Prakash Sangappa <prakash.sangappa@...cle.com> wrote:
>> For analysis purpose it is useful to have numa node information
>> corresponding mapped address ranges of the process. Currently
>> /proc/<pid>/numa_maps provides list of numa nodes from where pages are
>> allocated per VMA of the process. This is not useful if an user needs to
>> determine which numa node the mapped pages are allocated from for a
>> particular address range. It would have helped if the numa node information
>> presented in /proc/<pid>/numa_maps was broken down by VA ranges showing the
>> exact numa node from where the pages have been allocated.

I'm finding myself a little lost in figuring out what this does.  Today,
numa_maps might us that a 3-page VMA has 1 page from Node 0 and 2 pages
from Node 1.  We group *entirely* by VMA:

1000-4000 N0=1 N1=2

We don't want that.  We want to tell exactly where each node's memory is
despite if they are in the same VMA, like this:

1000-2000 N1=1
2000-3000 N0=1
3000-4000 N1=1

So that no line of output ever has more than one node's memory.  It
*appears* in this new file as if each contiguous range of memory from a
given node has its own VMA.  Right?

This sounds interesting, but I've never found myself wanting this
information a single time that I can recall.  I'd love to hear more.

Is this for debugging?  Are apps actually going to *parse* this file?

How hard did you try to share code with numa_maps?  Are you sure we
can't just replace numa_maps?  VMAs are a kernel-internal thing and we
never promised to represent them 1:1 in our ABI.

Are we going to continue creating new files in /proc every time a tiny
new niche pops up? :)