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

[Joel Fernandes <joel@joelfernandes.org>]

On Thu, Sep 05, 2019 at 01:47:05PM -0400, Joel Fernandes wrote:
> On Thu, Sep 05, 2019 at 01:35:07PM -0400, Steven Rostedt wrote:
> > 
> > 
> > [ Added Tom ]
> > 
> > On Thu, 5 Sep 2019 09:03:01 -0700
> > Suren Baghdasaryan <surenb@google.com> wrote:
> > 
> > > On Thu, Sep 5, 2019 at 7:43 AM Michal Hocko <mhocko@kernel.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@kernel.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?
> 
> 
> Hey Steve,
> 
> Something like an analog to digitial coversion function where you lose the
> granularity of the signal depending on how much trace data:
> https://www.globalspec.com/ImageRepository/LearnMore/20142/9ee38d1a85d37fa23f86a14d3a9776ff67b0ec0f3b.gif

s/how much trace data/what the resolution is/

> so like, if you had a counter incrementing with values after the increments
> as:  1,3,4,8,12,14,30 and say 5 is the threshold at which to emit a trace,
> then you would get 1,8,12,30.
> 
> So I guess what is need is a way to reduce the quantiy of trace data this
> way. For this usecase, the user mostly cares about spikes in the counter
> changing that accurate values of the different points.

s/that accurate/than accurate/

I think Tim, Suren, Dan and Michal are all saying the same thing as well.

thanks,

 - Joel