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

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Date:   Mon, 19 Sep 2016 21:56:20 +0200
From:   Jann Horn <jann@...jh.net>
To:     Michal Hocko <mhocko@...nel.org>
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, Sep 19, 2016 at 09:51:13PM +0200, Michal Hocko wrote:
> [not sure why the CC list was trimmed - do no do that please unless you
>  have a strong reason for that - if this was not intentional please
>  restpre it]

Ah, sorry, pressed the wrong key.


> On Mon 19-09-16 21:40:01, Jann Horn wrote:
> > On Mon, Sep 19, 2016 at 09:32:38PM +0200, Michal Hocko wrote:
> > > 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.
> > 
> > That sounds horrible. If some application decides that they want to check
> > statm directly after some action or so (like after program startup), this is
> > going to give them a very bad time. That probably doesn't happen
> > often - but still.
> > 
> > I can already imagine some developer going "yeah, that usleep()... that's
> > because the kernel API returns stale information for a couple milliseconds
> > after we do something *shrug*".
> > 
> > What are you trying to optimize for? Ten users on the same machine, each of
> > which is running "top" because it looks so great?
> 
> Please try to read what I wrote again. I didn't say this would be
> needed. The idea was that _if_ /proc/<pid>/statm is used very _often_
> than some caching might help to reduce the overhead. Especially when you
> consider that the information is not precise anyway. It can change
> anytime while you are doing the address space walk.

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