linux-kernel - Re: [PATCH 1/2] perf ftrace: Check min/max latency only with bucket range

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Message-ID: <Z4Bt8aIkX5uErX8a@google.com>
Date: Thu, 9 Jan 2025 16:46:41 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Gabriele Monaco <gmonaco@...hat.com>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Kan Liang <kan.liang@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	linux-perf-users@...r.kernel.org
Subject: Re: [PATCH 1/2] perf ftrace: Check min/max latency only with bucket
 range

On Thu, Jan 09, 2025 at 08:53:02AM +0100, Gabriele Monaco wrote:
> On Wed, 2025-01-08 at 13:00 -0800, Namhyung Kim wrote:
> > It's an optional feature and remains 0 when bucket range is not
> > given.
> > And it makes the histogram goes to the last entry always because any
> > latency (num) is greater than or equal to 0.
> 
> Thanks Namhyung for fixing this, something definitely slipped while
> testing..
> 
> I confirm your patches work well also when the bucket range is provided but the
> min latency isn't.
> 
> I'm wondering if it would be cleaner to propagate your changes (using
> min/max latency only if bucket_range is provided) also to
> make_histogram. That function currently works since we assume
> min_latency to be always 0, which is the case but probably not
> considering it altogether would look a bit better and prevent some
> headache in the future.

It looks good.  One thing I concern is 'num += min_latency' before
do_inc.  I put it there to make it symmetric to 'num -= min_latency'
so it should go to inside the block too.

Or you could factor it out as a function like 'i = get_bucket_index(num)'
so that it can keep the original num for the stats.

Thanks,
Namhyung

> 
> ---
>  tools/perf/builtin-ftrace.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)
> 
> diff --git a/tools/perf/builtin-ftrace.c b/tools/perf/builtin-ftrace.c
> index 076c703e5c334..82d63d7af9d03 100644
> --- a/tools/perf/builtin-ftrace.c
> +++ b/tools/perf/builtin-ftrace.c
> @@ -779,19 +779,16 @@ static void make_histogram(struct perf_ftrace *ftrace, int buckets[],
>  		if (ftrace->use_nsec)
>  			num *= 1000;
>  
> -		i = 0;
> -		if (num < min_latency)
> -			goto do_inc;
> -
> -		num -= min_latency;
> -
>  		if (!ftrace->bucket_range) {
>  			i = log2(num);
>  			if (i < 0)
>  				i = 0;
>  		} else {
> -			// Less than 1 unit (ms or ns), or, in the future,
> -			// than the min latency desired.
> +			i = 0;
> +			if (num < min_latency)
> +				goto do_inc;
> +
> +			num -= min_latency;
>  			if (num > 0) // 1st entry: [ 1 unit .. bucket_range units ]
>  				i = num / ftrace->bucket_range + 1;
>  			if (num >= max_latency - min_latency)
> 
> base-commit: 257ccfaf9fbef6f54eabf1a8f8a8efcc9036439b
> -- 
> 2.47.1
>