Re: [patch 0/3] [Announcement] Performance Counters for Linux

From: Ingo Molnar
Date: Fri Dec 05 2008 - 01:10:56 EST

* David Miller <davem@xxxxxxxxxxxxx> wrote:

> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Date: Thu, 04 Dec 2008 23:44:39 -0000
> > - No interaction with ptrace: any task (with sufficient permissions) can
> > monitor other tasks, without having to stop that task.
> This isn't going to work.
> If you look at the things the perfmon libraries do, you do need to stop
> the task.
> Consider counter virtualization as the most direct example. [...]

Note that counter virtualization is not offered in the perfmon3 patchset
that has been posted to lkml. (It is part of the much larger 'full'
perfmon patchset which has not been submitted for integration)

Nevertheless we will offer counter virtualization in -v2 of our patchset
and we mentioned it in the TODO list:

> > - Round-robin scheduling of counters, when there's more task
> > counters than hw counters available.

The 'target' task does not have to be stopped to offer counter
virtualization (counter overcommit or counter scheduling) - or to offer
any of the other performance counter features. Please let us know why it
needs the task to be stopped - we asked about that on lkml in the perfmon
thread and no technical answer was given, and couldnt find any such
technical reason while implementing it ourselves.

Relying on ptrace machinery can be considered one of the bigger design
mistakes of the permon3 patchset.

We pointed that out in review, and now we demonstrate it via this
patchset that it can be done much cleaner and much simpler. (Please stay
tuned for -v2 if you want to see the proof of the pudding.)

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