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

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Date:	Fri, 23 Jan 2015 00:28:38 +0000
From:	Primiano Tucci <primiano@...omium.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Petr Cermak <petrcermak@...omium.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	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 Thu, Jan 22, 2015 at 11:27 PM, David Rientjes <rientjes@...gle.com> wrote:
> If you reset the hwm for a process, rss grows to 100MB, another process
> resets the hwm, and you see a hwm of 2MB, that invalidates the hwm
> entirely.

Not sure I follow this scenario. Where does the 2MB come from? How can
you see a hwm of 2MB, under which conditions? HVM can never be < RSS.
Again, what you are talking about is the case of two profilers racing
for using the same interface (hwm).
This is the same case today of the PG_referenced bit.

> The hwm is already defined as the
> highest rss the process has attained, resetting it and trying to make any
> inference from the result is racy and invalidates the actual value which
> is useful.
The counter arugment is: once you have one very high peak, the hvm
becomes essentially useless for the rest of the lifetime of the
process (until a higher peak comes). This makes very hard to
understand what is going on in the meanwhile (from userspace).

Anyways, are you proposing to pursue a different approach? Is the
approach 2. that petrcermark@ proposed in the beginning of the thread
going to address this concern?
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