Messages in this thread | | | Subject | RE: [PATCH V4] POWER: perf_event: Skip updating kernel counters if register value shrinks | Date | Wed, 27 Apr 2011 08:40:36 +0100 | From | "David Laight" <> |
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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) & 0xfffffffful; > + > + /* > + * POWER7 can roll back counter values, if the new value is smaller > + * than the previous value it will cause the delta and the counter to > + * have bogus values unless we rolled a counter over. If a coutner is > + * rolled back, it will be smaller, but within 256, which is the maximum > + * number of events to rollback at once. If we dectect a rollback > + * return 0. This can lead to a small lack of precision in the > + * counters. > + */ > + if (prev > val && (prev - val) < 256) > + delta = 0; > + > + return delta;
The code should detect rollback by looking at the value of 'delta' otherwise there are horrid end effects near 2^32-1.
For instance: u32 delta = val - prev; return delta & 0x80000000 ? 0 : delta;
David
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