Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown metrics
From: Liang, Kan
Date: Mon Sep 09 2019 - 09:40:31 EST
On 8/31/2019 5:19 AM, Peter Zijlstra wrote:
But not for ever read, only on METRIC_OVF.
Then there is no mucking about with that odd counter/metrics msr pair
You have to write them to reset the internal counters.
reset nonsense. Becuase that really stinks.
The precision are lost if the counters were running much longer than the
measured delta. We have to reset the counters after every read.
For example, the worst case is:
The previous reading was rounded up and the current reading is rounded
down. The error of METRICS is 1/256 - 1/SLOTS, which is related to
SLOTS. The error for each Topdown event is (1/256 - 1/SLOTS) * SLOTS.
With the SLOTS increasing, the precision will get worse and worse.
Therefor, we have to reset the counters periodically.