Re: get_irq_regs() from soft IRQ

[Jean Pihet <jpihet@mvista.com>]

On Monday 29 June 2009 19:54:23 Siarhei Siamashka wrote:
> On Monday 29 June 2009 19:58:41 ext Jean Pihet wrote:
> > Hi Siarhei Siamashka,
> >
> > On Monday 29 June 2009 18:36:57 Siarhei Siamashka wrote:
> > > On Monday 29 June 2009 17:31:18 ext Jean Pihet wrote:
> > > > Hi,
> > > >
> > > > 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?
> > >
> > > Are you trying to implement (in a clean way) something similar to
> > > http://marc.info/?l=oprofile-list&m=123688347009580&w=2
> > >
> > > Or is it going to be a different workaround?
> >
> > I am trying to get a different approach, starting from the errata
> > description. The idea is to avoid the counters from overflowing, which
> > could cause a PMNC unit reset or lock-up (or both).
> >
> > Here are the implementation details:
> > - use a timer to read and reset the counters, then fire a work queue
> > - in the work queue the counters values are converted to oprofile samples
> > - the proper locking is used to avoid some races between the various
> > tasks
> >
> > I am nearly done with it but I am now running into problems with PM
> > (suspend/resume) and get_irq_regs().
> >
> > What do you think?
>
> Russel was the first to reply :)
>
> But we also discussed this "hybrid model" some time ago, and there is a
> clear counterexample where it fails:
> http://www.nabble.com/Re%3A--PATCH-0-1--OMAP-gptimer-based-event-monitor-dr
>iver-for-oprofile-p21374285.html
All right, sorry I was not aware of that discussion. So the PMNC unit is 
broken beyond repair. BTW good description and test results!

> > How far are you on your side? Did you stress test the solution? Is the
> > PMNC recovery always successful?
>
> I ended up just using a timer with high frequency of samples generation. it
> works without hassle and is sufficient for the majority of cases.
Ok. It looks like it is the best we can do.

Thanks,
Jean