Re: [Question] Hooks for scheduler tracing (CFS)

["Frank Ch. Eigler" <fche@redhat.com>]

Hi -

On Thu, Jul 26, 2007 at 11:02:26AM -0400, Mathieu Desnoyers wrote:
> [...]
> > > The problem is also in _stp_print_flush, not *only* in relay code:
> > > void _stp_print_flush (void)
> > > ...
> > >                 spin_lock(&_stp_print_lock);
> > >                 spin_unlock(&_stp_print_lock);
> > > 
> > > Those will turn into mutexes with -rt.
> > 
> > Indeed,

(Though actually that bug was fixed some time ago.)


> > plus systemtap-generated locking code uses rwlocks,
> > local_irq_save/restore or preempt_disable, in various places.  Could
> > someone point to a place that spells out what would be more
> > appropriate way of ensuring atomicity while being compatible with -rt?
> 
> AFAIK, for your needs either:
> [...]
> - Use per-cpu data with preempt disabling/irq disabling

As in local_irq_save / preempt_disable?  Yes, already done.

> - Use the original "real" spin locks/rwlocks (raw_*).
> [...]

It was unclear from the OLS paper whether the spin_lock_irq* family of
functions also had to be moved to the raw forms.

> You just don't want to sleep in the tracing code. [...]  Since you
> will likely disable preemption, make sure your tracing code executes
> in a deterministic time.

Definitely, that has always been the case.

> Make sure that the sub-buffer switch code respects that too [...]

We will review that part of the related code.

- FChE