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

[Peter Zijlstra <peterz@infradead.org>]

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 Krčmář 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.

> >  Qais did something very similar recently:
> > 
> > https://lore.kernel.org/lkml/20190604111459.2862-1-qais.yousef@arm.com/
> > 
> > You'll have to implement the associated trace events in a module, which
> > lets you define your own event format and doesn't form an ABI :).
> 
> Is that really true? eBPF programs loaded from userspace can access
> tracepoints through BPF_RAW_TRACEPOINT_OPEN, which is UAPI:
> https://github.com/torvalds/linux/blob/master/include/uapi/linux/bpf.h#L103
> 
> I don't have a strong opinion about considering tracepoints as ABI / API or
> not, but just want to get the facts straight :)

eBPF can access all sorts of kernel internals; if we were to deem eBPF
and API we'd be fscked.