Re: [RFC 4/5] x86, perf: implements lwp-perf-integration (rc1)

Ingo Molnar
2011-12-20T18:30:11Z

* Vince Weaver <vweaver1@eecs.utk.edu> wrote:

> On Tue, 20 Dec 2011, Ingo Molnar wrote:

> > Granted, LWP was mis-designed to quite a degree, those AMD
> > chip engineers should have talked to people who understand
> > how modern PMU abstractions are added to the OS kernel
> > properly.
>
> You do realize that LWP was probably in design 5+ years ago,
> at a time when most Linux kernel developers wanted nothing to
> do with perf counters, and thus anyone they did contact for
> help would have been from the since-rejected perfctr or
> perfmon2 camp.

That does not really contradict what i said.

> Also, I'm sure Linux isn't the only Operating System that they
> had in mind when designing this functionality.
>
> Running LWP through the kernel is a foolish idea. Does anyone
> have any numbers on what that would do to overhead?

At most an LLWPCB instruction is needed.

> perf_events creates huge overhead when doing self monitoring.
> For simple self-monintoring counter reads it is an *order of
> magnitude* worse than doing the same thing with perfctr.

Only if you are comparing apples to oranges: if you compare a
full kernel based read of self-profiling counters with an RDPMC
instruction.

But as we told you previously, you could use RDPMC under perf as
well, last i checked PeterZ posted experimental patches for
that. Peter, what's the status of that?

Thanks,

Ingo