Re: get_irq_regs() from soft IRQ

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Mon, Jun 29, 2009 at 07:36:57PM +0300, Siarhei Siamashka wrote:
> On Monday 29 June 2009 17:31:18 ext Jean Pihet wrote:
> > I am trying to get the latest IRQ registers from a timer or a work queue
> > but I am running into problems:
> > - get_irq_regs() returns NULL in some cases, so it is unsuable and even
> > causes crash when trying to get the registers values from the returned ptr
> > - I never get user space registers, only kernel
> >
> > The use case is that the performance unit (PMNC) of the Cortex A8 has some
> > serious bug, in short the performance counters overflow IRQ is to be
> > avoided. The solution I am implementing is to read and reset the counters
> > from a work queue that is triggered by a timer.
> 
> Regarding this oprofile related part. I wonder how you can get oprofile
> working properly (providing non-bogus results) without performance
> counters overflow IRQ generation? 

I don't think you can - triggering capture on overflow is precisely how
oprofile works.

The erratum talks about polling for overflow.  By doing this, you are in
a well defined part of the kernel, which is obviously going to be shown
as a hot path for every counter, thus making oprofile useless for kernel
work.

Deferring the interrupt to a workqueue doesn't resolve the problem either.
The problem has nothing to do with what happens after the interrupt
occurs - it's about interrupts themselves being lost.

I think just accepting that this erratum breaks oprofile is the only
realistic solution. ;(