Re: release 0.2 of x86 performance-monitoring counters support patch

Andi Kleen (ak@muc.de)
Thu, 10 Jun 1999 17:28:30 +0200

On Thu, Jun 10, 1999 at 10:00:39AM +0200, Mikael Pettersson wrote:
> Andi Kleen writes:
> > Only thing that I'm missing in your nice patch is a way to access the performance
> > counters of remote processes (so that I could hack gdb to do non intrusive
> > profiling a la itune). Extending the ioctls could work, although a ptrace
> > extension would be nicer.
>
> I know. I'm currently working on the design of this feature. I will probably
> represent the counter state objects as files in a pseudo filesystem, in order to
> get sharing, lifetimes [the state must survive process death if there's another
> process controlling it], and mmap() access right. SMP will be also have to be
> considered, but I guess I can steal code from ptrace() or kill().

Yuck, that sounds far too complicated and slow. I would like the interface
as lightweight as possible to avoid stealing too much cache from the debugee.
What is wrong with a simple ptrace extension to read/write the performance
registers?

The lifetime problem is already solved by ptrace (the debugee is reparented
to the debugger and gets a SIGCHLD and can still access the zombie in a limited
fashion)

ptrace basically does not care about SMP, because a ptraced process is stopped
and fortunately cannot move CPUs while it is inspecting it. For virtual
counters the SMP problems are handled for free in the normal context switch
(no IPIs needed like for global ones)

-Andi

-- 
This is like TV. I don't like TV.


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