Re: [PATCH v2 2/2] task_mmu: Add user-space support for resetting mm->hiwater_rss (peak RSS)

From: Primiano Tucci <primiano@chromium.org>
To: David Rientjes <rientjes@google.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>,
	Petr Cermak <petrcermak@chromium.org>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH v2 2/2] task_mmu: Add user-space support for resetting mm->hiwater_rss (peak RSS)
Date: Fri, 23 Jan 2015 00:28:38 +0000	[thread overview]
Message-ID: <CA+yH71e2ewvA41BNyb=TTPn+yx2zWzY6rn09hRVVgWKoeMgwXQ@mail.gmail.com> (raw)
In-Reply-To: <alpine.DEB.2.10.1501221523390.27807@chino.kir.corp.google.com>

On Thu, Jan 22, 2015 at 11:27 PM, David Rientjes <rientjes@google.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?