Messages in this thread | | | From | "" <> | Subject | Re: [PATCH 1/3] libperf: Add processing to scale the counters obtained during the read() system call when multiplexing | Date | Tue, 24 Aug 2021 10:11:28 +0000 |
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Hi, Rob
> On Fri, Aug 20, 2021 at 06:39:06PM +0900, Shunsuke Nakamura wrote: > > perf_evsel__read() scales counters obtained by RDPMC during multiplexing, but > > does not scale counters obtained by read() system call. > > > > Add processing to perf_evsel__read() to scale the counters obtained during the > > read() system call when multiplexing. > > Which one is right though? Changing what read() returns could break > users, right? Or are you implying that the RDPMC path is correct and > read() was not. More likely the former case since I wrote the latter.
perf_evsel__read() returns both the count obtained by RDPMC and the count obtained by the read() system call when multiplexed with RDPMC enabled.
That is, there is a mix of scaled and unscaled values.
As Rob says, when this patch is applied, rescaling the count obtained from perf_evsel__read() during multiplexing will break the count.
I think the easiest solution is to change the value you get from RDPMC to not scale and let the user scale it, but I thought it would be a little inconvenient.
Best Regards Shunsuke
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