linux-kernel - Re: [PATCH v5 0/3] mm, proc: Implement /proc/<pid>/totmaps

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Date:   Mon, 19 Sep 2016 21:32:38 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Robert Foss <robert.foss@...labora.com>
Cc:     Sonny Rao <sonnyrao@...omium.org>,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Hugh Dickins <hughd@...gle.com>,
        Konstantin Khlebnikov <koct9i@...il.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Stultz <john.stultz@...aro.org>,
        Minchan Kim <minchan@...nel.org>, ross.zwisler@...ux.intel.com,
        jmarchan@...hat.com, Johannes Weiner <hannes@...xchg.org>,
        Kees Cook <keescook@...omium.org>, oleg@...hat.com,
        Al Viro <viro@...iv.linux.org.uk>,
        Mateusz Guzik <mguzik@...hat.com>,
        Janis Danisevskis <jdanis@...gle.com>, calvinowens@...com,
        Alexey Dobriyan <adobriyan@...il.com>, ebiederm@...ssion.com,
        Seth Forshee <seth.forshee@...onical.com>,
        Djalal Harouni <tixxdz@...il.com>, linux-doc@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ben Zhang <benzh@...omium.org>,
        Bryan Freed <bfreed@...omium.org>,
        Filipe Brandenburger <filbranden@...omium.org>,
        Jann Horn <jann@...jh.net>, linux-api@...r.kernel.org,
        Jacek Anaszewski <j.anaszewski@...sung.com>
Subject: Re: [PATCH v5 0/3] mm, proc: Implement /proc/<pid>/totmaps

On Mon 19-09-16 11:16:31, Robert Foss wrote:
> On 2016-09-14 05:12 AM, Michal Hocko wrote:
> > On Tue 13-09-16 13:27:39, Sonny Rao wrote:
[...]
> > > Given that smaps
> > > doesn't provide this in a straightforward way, what do you think is
> > > the right way to provide this information?
> > 
> > I would be tempted to sneak it into /proc/<pid>/statm because that looks
> > like a proper place but getting this information is not for free
> > performance wise so I am not really sure something that relies on this
> > file would see unexpected stalls. Maybe this could be worked around by
> > some caching... I would suggest to check who is actually using this file
> > (top/ps etc...)
> 
> What would this caching look like? Can any information be re-used between
> vma walks?

yes basically return the same value if called within HZ or something
similar. But that assumes that statm latency really matters and it is
called often enough.

-- 
Michal Hocko
SUSE Labs