Re: get_irq_regs() from soft IRQ

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

On Mon, Jun 29, 2009 at 06:58:41PM +0200, Jean Pihet wrote:
> 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).

But this can't work.

Oprofile essentially works as follows:

  You set the number (N) of events you wish to occur between each sample.
  When N events have occured, you record the stacktrace and reset the
  counter so it fires after another N events.

Now, you could start the counters at zero every time, and then poll them
via a timer.  When the counter value is larger than N, you could log a
stacktrace and zero the counter.

However, this suffers one very serious problem - if you're wanting to
measure something at an interval which occurs faster than your timer,
you're going to get misleading results.

You could set the timer to fire at a high rate, but then that's going
to upset things like cache miss, cache hit, etc measurements.

> 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

This sounds over complicated.  I see no reason for a workqueue to be
involved anywhere near the oprofile sample code.

> I am nearly done with it but I am now running into problems with PM 
> (suspend/resume) and get_irq_regs().

You really really really can't use get_irq_regs() outside of IRQ context.
The stored registers just do not exist anymore - they've been overwritten
by whatever exception or system call you're currently in.

You can't create a copy of them - copies will be overwritten on the very
next (nested) interrupt.  You don't know which interrupt is the first
interrupt to occur.

I really think that the only option here is to just accept that oprofile
is crucified by this errata.