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

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Date:   Thu, 5 Sep 2019 13:35:07 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Tim Murray <timmurray@...gle.com>,
        Carmen Jackson <carmenjackson@...gle.com>,
        Mayank Gupta <mayankgupta@...gle.com>,
        Daniel Colascione <dancol@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kernel-team <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 <linux-mm@...ck.org>,
        Matthew Wilcox <willy@...radead.org>,
        Ralph Campbell <rcampbell@...dia.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Tom Zanussi <zanussi@...nel.org>
Subject: Re: [PATCH v2] mm: emit tracepoint when RSS changes by threshold



[ Added Tom ]

On Thu, 5 Sep 2019 09:03:01 -0700
Suren Baghdasaryan <surenb@...gle.com> wrote:

> On Thu, Sep 5, 2019 at 7:43 AM Michal Hocko <mhocko@...nel.org> wrote:
> >
> > [Add Steven]
> >
> > 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:  
> > [...]  
> > > > > but also for reducing
> > > > > tracing noise. Flooding the traces makes it less useful for long traces and
> > > > > post-processing of traces. IOW, the overhead reduction is a bonus.  
> > > >
> > > > This is not really anything special for this tracepoint though.
> > > > Basically any tracepoint in a hot path is in the same situation and I do
> > > > not see a point why each of them should really invent its own way to
> > > > throttle. Maybe there is some way to do that in the tracing subsystem
> > > > directly.  
> > >
> > > I am not sure if there is a way to do this easily. Add to that, the fact that
> > > you still have to call into trace events. Why call into it at all, if you can
> > > filter in advance and have a sane filtering default?
> > >
> > > The bigger improvement with the threshold is the number of trace records are
> > > almost halved by using a threshold. The number of records went from 4.6K to
> > > 2.6K.  
> >
> > Steven, would it be feasible to add a generic tracepoint throttling?  
> 
> I might misunderstand this but is the issue here actually throttling
> of the sheer number of trace records or tracing large enough changes
> to RSS that user might care about? Small changes happen all the time
> but we are likely not interested in those. Surely we could postprocess
> the traces to extract changes large enough to be interesting but why
> capture uninteresting information in the first place? IOW the
> throttling here should be based not on the time between traces but on
> the amount of change of the traced signal. Maybe a generic facility
> like that would be a good idea?

You mean like add a trigger (or filter) that only traces if a field has
changed since the last time the trace was hit? Hmm, I think we could
possibly do that. Perhaps even now with histogram triggers?

-- Steve