On Wed, 2011-04-27 at 08:40 +0100, David Laight wrote:
> I keep telling Eric that the code below is incorrect
> modulo arithimetic...
His previous versions were wrong yes. This one should be limping along
afaik. But I tend to agree, testing delta is the way to go.
Eric idea was to not test the si
On Wed, 27 Apr 2011, David Laight wrote:
>
> > prev and val are both 64 bit variables holding 32 bit numbers, we do
> not
> > accumulate in either, they are both replaced by values directly from
> the
> > registers.
> > So prev > val will not always be true.
>
> The code seems to be:
> prev
> prev and val are both 64 bit variables holding 32 bit numbers, we do
not
> accumulate in either, they are both replaced by values directly from
the
> registers.
> So prev > val will not always be true.
The code seems to be:
prev = local64_read(&event->hw.prev_count);
val = read_pmc(eve
On Wed, 27 Apr 2011, David Laight wrote:
>
> > + if (prev > val && (prev - val) < 256)
> > + delta = 0;
> > +
> > + return delta;
>
> Also, 'prev' is a true 64bit value, but 'val' is only ever 32bits.
> So once the 64bit 'prev' exceeds 2^32+256 both 'prev > val'
> and 'prev - val'
> On Wed, 27 Apr 2011, David Laight wrote:
>
> > > But it isn't, and it doesn't have trouble with 2^32 - 1.
> >
> > what about:
> > prev = 0x0001
> > val = 0x
>
> Result is 0xfffe and we are fine.
'delta' will be 0xfffe, but you need the function to
return zero - since
On Wed, 27 Apr 2011, David Laight wrote:
> > But it isn't, and it doesn't have trouble with 2^32 - 1.
>
> what about:
> prev = 0x0001
> val = 0x
Result is 0xfffe and we are fine.
>
> David
>
>
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> + if (prev > val && (prev - val) < 256)
> + delta = 0;
> +
> + return delta;
Also, 'prev' is a true 64bit value, but 'val' is only ever 32bits.
So once the 64bit 'prev' exceeds 2^32+256 both 'prev > val'
and 'prev - val' are true regardless of the value of 'val'.
This will
> But it isn't, and it doesn't have trouble with 2^32 - 1.
what about:
prev = 0x0001
val = 0x
David
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On Wed, 27 Apr 2011, David Laight wrote:
> I keep telling Eric that the code below is incorrect
> modulo arithimetic...
But it isn't, and it doesn't have trouble with 2^32 - 1. Here is one done by
hand:
Counter is at 0x and is rolled over to 0x101 (258 counted items so
that we miss the
I keep telling Eric that the code below is incorrect
modulo arithimetic...
> +static u64 check_and_compute_delta(u64 prev, u64 val)
> +{
> + u64 delta = (val - prev) & 0xul;
> +
> + /*
> + * POWER7 can roll back counter values, if the new value is
smaller
> + * than the p
Because of speculative event roll back, it is possible for some event coutners
to decrease between reads on POWER7. This causes a problem with the way that
counters are updated. Delta calues are calculated in a 64 bit value and the
top 32 bits are masked. If the register value has decreased, thi
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