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

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Date:	Thu, 26 Jul 2007 11:02:26 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"Frank Ch. Eigler" <fche@...hat.com>
Cc:	Ankita Garg <ankita@...ibm.com>,
	Arjan van de Ven <arjan@...radead.org>, linux@...mer.net,
	LKML <linux-kernel@...r.kernel.org>,
	RT-Users <linux-rt-users@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [Question] Hooks for scheduler tracing (CFS)

* Frank Ch. Eigler (fche@...hat.com) wrote:
> Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> writes:
> 
> > [...]
> > 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, 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?
> 
> - FChE

AFAIK, for your needs either:
- Use atomic ops
- Use per-cpu data with preempt disabling/irq disabling
- Use the original "real" spin locks/rwlocks (raw_*).
- Don't play with timers or wakeups, since this kernel code uses the
  "standard" spin locks (sleepable in -rt).

You just don't want to sleep in the tracing code.

Make sure that the sub-buffer switch code respects that too: it is the
most tricky and yet less executed part of the tracing code, so it's easy
for bugs to slip there and yet be undetected for a while.

Since you will likely disable preemption, make sure your tracing code
executes in a deterministic time.

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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