Re: [PATCH 2/2] sched/debug: add sched_update_nr_running tracepoint

[Qais Yousef]

On 09/04/19 10:14, Peter Zijlstra wrote:
> On Wed, Sep 04, 2019 at 12:23:10AM -0400, Joel Fernandes wrote:
> > On Tue, Sep 03, 2019 at 05:05:47PM +0100, Valentin Schneider wrote:
> > > On 03/09/2019 16:43, Radim KrÄmÃÅ wrote:
> > > > The paper "The Linux Scheduler: a Decade of Wasted Cores" used several
> > > > custom data gathering points to better understand what was going on in
> > > > the scheduler.
> > > > Red Hat adapted one of them for the tracepoint framework and created a
> > > > tool to plot a heatmap of nr_running, where the sched_update_nr_running
> > > > tracepoint is being used for fine grained monitoring of scheduling
> > > > imbalance.
> > > > The tool is available from https://github.com/jirvoz/plot-nr-running.
> > > > 
> > > > The best place for the tracepoints is inside the add/sub_nr_running,
> > > > which requires some shenanigans to make it work as they are defined
> > > > inside sched.h.
> > > > The tracepoints have to be included from sched.h, which means that
> > > > CREATE_TRACE_POINTS has to be defined for the whole header and this
> > > > might cause problems if tree-wide headers expose tracepoints in sched.h
> > > > dependencies, but I'd argue it's the other side's misuse of tracepoints.
> > > > 
> > > > Moving the import sched.h line lower would require fixes in s390 and ppc
> > > > headers, because they don't include dependecies properly and expect
> > > > sched.h to do it, so it is simpler to keep sched.h there and
> > > > preventively undefine CREATE_TRACE_POINTS right after.
> > > > 
> > > > Exports of the pelt tracepoints remain because they don't need to be
> > > > protected by CREATE_TRACE_POINTS and moving them closer would be
> > > > unsightly.
> > > > 
> > > 
> > > Pure trace events are frowned upon in scheduler world, try going with
> > > trace points. 
> 
> Quite; I hate tracepoints for the API constraints they impose. Been
> bitten by that, not want to ever have to deal with that again.

s/tracepoints/trace events/ ?

They used to be one and the same but I think using them interchangeably might
cause some confusion now since we have tracepoints without trace events
associated with them.

Not trying to be picky, but the missing distinction confused the hell out of
me when I first started looking at this :-)

--
Qais Yousef