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

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Message-ID: <YSQBI+FZLBvKr088@robh.at.kernel.org>
Date:   Mon, 23 Aug 2021 15:12:19 -0500
From:   Rob Herring <robh@...nel.org>
To:     Shunsuke Nakamura <nakamura.shun@...itsu.com>
Cc:     peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
        mark.rutland@....com, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, namhyung@...nel.org,
        linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org
Subject: Re: [PATCH 1/3] libperf: Add processing to scale the counters
 obtained during the read() system call when multiplexing

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.

> 
> Signed-off-by: Shunsuke Nakamura <nakamura.shun@...itsu.com>
> ---
>  tools/lib/perf/evsel.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/tools/lib/perf/evsel.c b/tools/lib/perf/evsel.c
> index d8886720e83d..005cf64a1ad7 100644
> --- a/tools/lib/perf/evsel.c
> +++ b/tools/lib/perf/evsel.c
> @@ -18,6 +18,7 @@
>  #include <sys/ioctl.h>
>  #include <sys/mman.h>
>  #include <asm/bug.h>
> +#include <linux/math64.h>
>  
>  void perf_evsel__init(struct perf_evsel *evsel, struct perf_event_attr *attr,
>  		      int idx)
> @@ -308,6 +309,9 @@ int perf_evsel__read(struct perf_evsel *evsel, int cpu, int thread,
>  	if (readn(FD(evsel, cpu, thread), count->values, size) <= 0)
>  		return -errno;
>  
> +	if (count->ena != count->run)
> +		count->val = mul_u64_u64_div64(count->val, count->ena, count->run);
> +
>  	return 0;
>  }
>  
> -- 
> 2.25.1
> 
>