So caveat that I'm ill and cannot think much.. On Mon, Nov 28, 2016 at 11:26:46AM -0800, [email protected] wrote:
> Here, all the possible failure cases are listed. > Terms: > - new: current PMU counter value which read from rdpmcl. > - prev: previous counter value which is stored in &hwc->prev_count. > - in PMI/not in PMI: the event update happens in PMI handler or not. > Current code to calculate delta: > delta = (new << shift) - (prev << shift); > delta >>= shift; > > Case A: Not in PMI. new > prev. But delta is negative. > That's the failure case of Test 2. > delta is s64 type. new and prev are u64 type. If the new is big > enough, after doing left shift and sub, the bit 64 of the result may > still be 1. > After converting to s64, the sign flag will be set. Since delta is > s64 type, arithmetic right shift is applied, which copy the sign flag > into empty bit positions on the upper end of delta. > It can be fixed by adding the max count value. > > Here is the real data for test2 on KNL. > new = aea96e1927 > prev = ffffff0000000001 > delta = aea96e1927000000 - 1000000 = aea96e1926000000 > aea96e1926000000 >> 24 = ffffffaea96e1926 << negative delta How can this happen? IIRC the thing increments, we program a negative value, and when it passes 0 we generate a PMI. And note that we _ALWAYS_ set the IN bits, even for !sampling events. Also note we set max_period to (1<<31) - 1, so we should never exceed 31 bits. > Case B: In PMI. new > prev. delta is positive. > That's the failure case of Test 3. > The PMI is triggered by overflow. But there is latency between > overflow and PMI handler. So new has small amount. > Current calculation lose the max count value. That doesn't make sense, per the 31bit limit. > Case C: In PMI. new < prev. delta is negative. > The PMU counter may be start from a big value. E.g. the fixed period > is small. > It can be fixed by adding the max count value. Doesn't make sense, how can this happen?
