Re: [RFC PATCH] perf_core: provide a kernel-internal interface to get to performance counters

[Ingo Molnar <mingo@elte.hu>]

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Thu, 2009-10-01 at 10:53 +0200, Ingo Molnar wrote:
> > i'd suggest you extend perf events with a 'system 
> > wide' event abstraction, which:
> > 
> >  - Enumerates such registered events (via a list)
> > 
> >  - Adds a CPU hotplug handler (which clones those events over to a new
> >    CPU and directs it back to the ring-buffer of the existing event(s)
> >    [if any])
> > 
> >  - Plus a state field that allows the filtering out of stray/premature
> >    events.
> > 
> > Such an add-on layer/abstraction would sure be useful in other cases as 
> > well. It might make sense to expose it to user-space and make perf top 
> > use it by default.
> 
> Non-trivial.
> 
> Something like this would imply a single output channel for all these 
> CPUs, and we've already seen that stuffing too many CPUs down one such 
> channel (using -M) leads to significant performance issues.

We could add internal per cpu buffering before it hits any globally 
visible output channel. (That has come up when i talked to Frederic 
about the function tracer.) We could even have page sized output (via 
the introduction of a NOP event that fills up to the next page edge).

This would have advantages elsewhere as well - it would be an immediate 
speedup for 'perf sched record' for example.

> Therefore I would strongly argue to let the kernel interface be what 
> it is and solve this in a userspace library for those who care.
> 
> We really cannot sanely support an all-CPUs abstraction without 
> running into trouble.

User-space will be in an even poorer situation to solve this 
intelligently.

Really, the only reason to _not_ abstract something in the kernel, 
_ever_ is when:

 - it is so trivial that it needs no extra helpers in the kernel

 - or when it is so specialized that it's a policy in essence

'it is too difficult' is a real _in favor_ of putting something into the 
kernel ;-)

	Ingo