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

[Joel Fernandes <joel@joelfernandes.org>]

On Thu, Sep 05, 2019 at 06:15:43PM -0700, Daniel Colascione wrote:
[snip]
> > > > > > > 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?
> > >
> > > I was thinking along the same lines. The histogram subsystem seems
> > > like a very good fit here. Histogram triggers already let users talk
> > > about specific fields of trace events, aggregate them in configurable
> > > ways, and (importantly, IMHO) create synthetic new trace events that
> > > the kernel emits under configurable conditions.
> >
> > Hmm, I think this tracing feature will be a good idea. But in order not to
> > gate this patch, can we agree on keeping a temporary threshold for this
> > patch? Once such idea is implemented in trace subsystem, then we can remove
> > the temporary filter.
> >
> > As Tim said, we don't want our traces flooded and this is a very useful
> > tracepoint as proven in our internal usage at Android. The threshold filter
> > is just few lines of code.
> 
> I'm not sure the threshold filtering code you've added does the right
> thing: we don't keep state, so if a counter constantly flips between
> one "side" of the TRACE_MM_COUNTER_THRESHOLD and the other, we'll emit
> ftrace events at high frequency. More generally, this filtering
> couples the rate of counter logging to the *value* of the counter ---
> that is, we log ftrace events at different times depending on how much
> memory we happen to have used --- and that's not ideal from a
> predictability POV.
> 
> All things being equal, I'd prefer that we get things upstream as fast
> as possible. But in this case, I'd rather wait for a general-purpose
> filtering facility (whether that facility is based on histogram, eBPF,
> or something else) rather than hardcode one particular fixed filtering
> strategy (which might be suboptimal) for one particular kind of event.
> Is there some special urgency here?
> 
> How about we instead add non-filtered tracepoints for the mm counters?
> These tracepoints will still be free when turned off.
> 
> Having added the basic tracepoints, we can discuss separately how to
> do the rate limiting. Maybe instead of providing direct support for
> the algorithm that I described above, we can just use a BPF program as
> a yes/no predicate for whether to log to ftrace. That'd get us to the
> same place as this patch, but more flexibly, right?

Chatted with Daniel offline, we agreed on removing the threshold -- which
Michal also wants to be that way.

So I'll be resubmitting this patch with the threshold removed; and we'll work
on seeing to use filtering through other generic ways like BPF.

thanks all!

 - Joel