Re: perf: prctl(PR_TASK_PERF_EVENTS_DISABLE) has no effect

[Ingo Molnar <mingo@kernel.org>]

* Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> > It also allows system-wide profiling after you've modified a 
> > library to self-profile, while your suggestion does not 
> > allow that.
> 
> But its no long self-profiling when some other process is 
> involved. And system wide is definitely not self.

If I'm a library developer and want to self-profile my 
modifications, then it's entirely appropriate to do a 
system-wide profile to see the system-wide use of this library!

System-wide and self-profiling is not exclusive. Think of it as 
a narrow, precise filter applied to a given area of 
functionality only.

AFAICS the alternative, under your method, would be to recompile 
every single app in the system - that's cumbersome beyond 
imagination, I wouldn't even call it a solution, let alone a 
quality implementation.

> > > It really isn't that hard to make userspace do what is 
> > > needed, it just takes a bit of work.
> > 
> > Even if your suggested solution was available (it isn't), my 
> > suggested approach is easier to use and covers more 
> > usecases.
> > 
> > User-space expecting the kernel to provide usable and 
> > minimal interfaces is not 'being silly'. It's the 
> > fundamental task of a kernel to provide them.
> 
> Bloating the interface for something that is already well 
> possible is.

There's no 'bloat' worth speaking off: a single bit out of an 
already allocated bitmap, plus a single check in an already 
existing loop, plus a single new command-line flag to the 
tooling side and minimal glue.

There's no runtime overhead to any other perf functionality, at 
all.

Thanks,

	Ingo