Re: RFC - Approaches to user-space probes

[Prasanna S Panchamukhi <prasanna@in.ibm.com>]

On Mon, Mar 27, 2006 at 09:37:48AM +0200, Arjan van de Ven wrote:
> On Mon, 2006-03-27 at 12:24 +0530, Prasanna S Panchamukhi wrote:
> 
> > - Low overhead and user can have thousands of active probes on the
> >   system and detect any instance when the probe was hit including
> >   probes on shared library etc.
> 
> I suspect this is the only reason for doing it inside the kernel;
> anything else still really shouts "do it in userspace via ptrace" to me.
> 

Other reasons would be:

- to view some privilaged data, such as system regs while you are
  debugging in user-space
- to view many arbitrary process address-space that use a common set
  of modules - user or kernel space

> 
> > ===========================================================
> > LOCAL PROBES(PER PROCESS) VS GLOBAL PROBES(EXECUTABLE FILE)
> > ===========================================================
> > 
> > - All processes take a trap since the same executable file
> >   gets mapped into different address_space.
> 
> is that true for breakpoints inserted after start?

Yes, insertion of the breakpoint happens at the physical
page level and it gets written back to the disc.

> The reason I ask because... what if half the processed took a COW on the
> page with the instruction you want to trap on. Are you going to edit all
> those COW'd pages?

The current prototype does not insert probes on COW pages, but yes eventually
we will provide probes insertions on COW'd pages feature too.

Thanks
Prasanna
-- 
Prasanna S Panchamukhi
Linux Technology Center
India Software Labs, IBM Bangalore
Email: prasanna@in.ibm.com
Ph: 91-80-51776329