linux-kernel - Re: [PATCH v2] mm: emit tracepoint when RSS changes by threshold

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Date:   Thu, 5 Sep 2019 16:20:10 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, Tim Murray <timmurray@...gle.com>,
        carmenjackson@...gle.com, mayankgupta@...gle.com,
        dancol@...gle.com, rostedt@...dmis.org, minchan@...nel.org,
        akpm@...ux-foundation.org, kernel-team@...roid.com,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jerome Glisse <jglisse@...hat.com>, linux-mm@...ck.org,
        Matthew Wilcox <willy@...radead.org>,
        Ralph Campbell <rcampbell@...dia.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v2] mm: emit tracepoint when RSS changes by threshold

On Thu 05-09-19 10:14:52, Joel Fernandes wrote:
> On Thu, Sep 05, 2019 at 12:54:24PM +0200, Michal Hocko wrote:
> > On Wed 04-09-19 12:28:08, Joel Fernandes wrote:
> > > On Wed, Sep 4, 2019 at 11:38 AM Michal Hocko <mhocko@...nel.org> wrote:
> > > >
> > > > On Wed 04-09-19 11:32:58, Joel Fernandes wrote:
> > > > > On Wed, Sep 04, 2019 at 10:45:08AM +0200, Michal Hocko wrote:
> > > > > > On Tue 03-09-19 16:09:05, Joel Fernandes (Google) wrote:
> > > > > > > Useful to track how RSS is changing per TGID to detect spikes in RSS and
> > > > > > > memory hogs. Several Android teams have been using this patch in various
> > > > > > > kernel trees for half a year now. Many reported to me it is really
> > > > > > > useful so I'm posting it upstream.
> > > > > > >
> > > > > > > Initial patch developed by Tim Murray. Changes I made from original patch:
> > > > > > > o Prevent any additional space consumed by mm_struct.
> > > > > > > o Keep overhead low by checking if tracing is enabled.
> > > > > > > o Add some noise reduction and lower overhead by emitting only on
> > > > > > >   threshold changes.
> > > > > >
> > > > > > Does this have any pre-requisite? I do not see trace_rss_stat_enabled in
> > > > > > the Linus tree (nor in linux-next).
> > > > >
> > > > > No, this is generated automatically by the tracepoint infrastructure when a
> > > > > tracepoint is added.
> > > >
> > > > OK, I was not aware of that.
> > > >
> > > > > > Besides that why do we need batching in the first place. Does this have a
> > > > > > measurable overhead? How does it differ from any other tracepoints that we
> > > > > > have in other hotpaths (e.g.  page allocator doesn't do any checks).
> > > > >
> > > > > We do need batching not only for overhead reduction,
> > > >
> > > > What is the overhead?
> > > 
> > > The overhead is occasionally higher without the threshold (that is if we
> > > trace every counter change). I would classify performance benefit to be
> > > almost the same and within the noise.
> > 
> > OK, so the additional code is not really justified.
> 
> It is really justified. Did you read the whole of the last email?

Of course I have. The information that numbers are in noise with some
outliers (without any details about the underlying reason) is simply
showing that you are optimizing something probably not worth it.

I would recommend adding a simple tracepoint. That should be pretty non
controversial. And if you want to add an optimization on top then
provide data to justify it.
-- 
Michal Hocko
SUSE Labs