linux-kernel - Re: [PATCH v2 2/2] task_mmu: Add user-space support for resetting mm->hiwater

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Date:	Wed, 14 Jan 2015 15:22:25 +0000
From:	Petr Cermak <petrcermak@...omium.org>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Primiano Tucci <primiano@...omium.org>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH v2 2/2] task_mmu: Add user-space support for resetting
 mm->hiwater_rss (peak RSS)

On Wed, Jan 07, 2015 at 07:24:52PM +0200, Kirill A. Shutemov wrote:
> And how it's not an ABI break?
I don't think this is an ABI break because the current behaviour is not
changed unless you write "5" to /proc/pid/clear_refs. If you do, you are
explicitly requesting the new functionality.

> We have never-lowering VmHWM for 9+ years. How can you know that nobody
> expects this behaviour?
This is why we sent an RFC [1] several weeks ago. We expect this to be
used mainly by performance-related tools (e.g. profilers) and from the
comments in the code [2] VmHWM seems to be a best-effort counter. If this
is strictly a no-go, I can only think of the following two alternatives:

  1. Add an extra resettable field to /proc/pid/status (e.g.
     resettable_hiwater_rss). While this doesn't violate the current
     definition of VmHWM, it adds an extra line to /proc/pid/status,
     which I think is a much bigger issue.
  2. Introduce a new proc fs file to task_mmu (e.g.
     /proc/pid/profiler_stats), but this feels like overengineering.

> And why do you reset hiwater_rss, but not hiwater_vm?
This is a good point. Should we reset both using the same flag, or
introduce a new one ("6")?

[1] lkml.iu.edu/hypermail/linux/kernel/1412.1/01877.html
[2] task_mmu.c:32: "... such snapshots can always be inconsistent."

Petr
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