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

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Date:	Thu, 15 Jan 2015 01:36:30 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Petr Cermak <petrcermak@...omium.org>
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 14, 2015 at 03:22:25PM +0000, Petr Cermak wrote:
> 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.

I'm not sure if it should be considered ABI break or not. Just asking.

I would like to hear opinion from other people.
 
> > 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.

I don't think extra line is bigger issue. Sane applications would look for
a key, not line number. We do add lines there. I've posted patch which
adds one more just today ;)

>   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."
-- 
 Kirill A. Shutemov
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