Re: oprofile and ARM A9 hardware counter

[Will Deacon <will.deacon@arm.com>]

On Thu, Feb 16, 2012 at 04:37:35PM +0000, Ming Lei wrote:
> On Fri, Feb 17, 2012 at 12:19 AM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> > On Fri, 2012-02-17 at 00:12 +0800, Ming Lei wrote:
> >> is triggered: u64 delta = 100 -  1000000 = 18446744073708551716.
> >
> > on x86 we do:
> >
> >  int shift = 64 - x86_pmu.cntval_bits;
> >  s64 delta;
> >
> >  delta = (new_raw_count << shift) - (prev_raw_count << shift);
> >  delta >>= shift;
> >
> > This deals with short overflows (on x86 the registers are typically 40
> > or 48 bits wide). If the arm register is 32 you can of course also get
> > there with some u32 casts.
> 
> Good idea, but it may not work if new_raw_count is bigger than prev_raw_count.

The more I think about this, the more I think that the overflow parameter to
armpmu_event_update needs to go. It was introduced to prevent massive event
loss in non-sampling mode, but I think we can get around that by changing
the default sample_period to be half of the max_period, therefore giving
ourselves a much better chance of handling the interrupt before new wraps
around past prev.

Ming Lei - can you try the following please? If it works for you, then I'll
do it properly and kill the overflow parameter altogether.

Thanks,

Will

git a/arch/arm/kernel/perf_event.c b/arch/arm/kernel/perf_event.c
index 5bb91bf..ef597a3 100644
--- a/arch/arm/kernel/perf_event.c
+++ b/arch/arm/kernel/perf_event.c
@@ -193,13 +193,7 @@ again:
                             new_raw_count) != prev_raw_count)
                goto again;
 
-       new_raw_count &= armpmu->max_period;
-       prev_raw_count &= armpmu->max_period;
-
-       if (overflow)
-               delta = armpmu->max_period - prev_raw_count + new_raw_count + 1;
-       else
-               delta = new_raw_count - prev_raw_count;
+       delta = (new_raw_count - prev_raw_count) & armpmu->max_period;
 
        local64_add(delta, &event->count);
        local64_sub(delta, &hwc->period_left);
@@ -518,7 +512,7 @@ __hw_perf_event_init(struct perf_event *event)
        hwc->config_base            |= (unsigned long)mapping;
 
        if (!hwc->sample_period) {
-               hwc->sample_period  = armpmu->max_period;
+               hwc->sample_period  = armpmu->max_period >> 1;
                hwc->last_period    = hwc->sample_period;
                local64_set(&hwc->period_left, hwc->sample_period);
        }