Messages in this thread | | | Subject | Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown metrics | From | "Liang, Kan" <> | Date | Wed, 28 Aug 2019 15:35:02 -0400 |
| |
On 8/28/2019 11:02 AM, Peter Zijlstra wrote: >> Reset >> ====== >> >> The PERF_METRICS and Fixed counter 3 have to be reset for each read, >> because: >> - The 8bit metrics ratio values lose precision when the measurement >> period gets longer. > So it musn't be too hot, > >> - The PERF_METRICS may report wrong value if its delta was less than >> 1/255 of SLOTS (Fixed counter 3).
The "delta" is actually for the internal counters. Sorry for the confusion.
> it also musn't be too cold. But that leaves it unspecified what exactly > is the right range. > > IOW, you want a Goldilocks number of SLOTS. > >> Also, for counting, the -max_period is the initial value of the SLOTS. >> The huge initial value will definitely trigger the issue mentioned >> above. Force initial value as 0 for topdown and slots event counting. > But you just told us that 0 is wrong too (too cold).
We set -max_period (0x8000 0000 0001) as initial value of FIXCTR3 for counting. But the internal counters probably starts counting from 0. PERF_METRICS is fraction of internal counters / FIXCTR3. So PERF_METRICS will never works. We have to make FIXCTR3 count from 0 as well.
Thanks, Kan
| |