Re: [PATCH 1/3] libperf: Add processing to scale the counters obtained during the read() system call when multiplexing

nakamura.shun
2021-09-17T08:09:35Z

Hi, Ian

> > On Tue, Aug 24, 2021 at 5:12 AM nakamura.shun@fujitsu.com
> > <nakamura.shun@fujitsu.com> wrote:
> > >
> > > 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.
> >
> > Agreed, unless someone else has an opinion. It would be good to do the
> > scaling in libperf with the optimized math op, but I assume there's
> > some reason the user may need unscaled values?
>
> Hi, something I've mentioned on other threads [1] is that running may
> be zero due to multiplexing but enabled be greater.

Thanks for your comment.
I'll fix it.

> This can lead to a divide by zero when scaling. Giving the ratio to the caller
> gives more information - I may be misunderstanding this thread, apologies if so.

The perf_counts_values contains enabled and running.
So, caller can calculate the ratio.

Best Regards
Shunsuke