linux-kernel - Re: [PATCH 3/3] perf report: Add weight[123] output fields

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Date: Tue, 9 Apr 2024 09:53:41 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: "Liang, Kan" <kan.liang@...ux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>, Ian Rogers <irogers@...gle.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, Stephane Eranian <eranian@...gle.com>, 
	Andi Kleen <ak@...ux.intel.com>, Athira Rajeev <atrajeev@...ux.vnet.ibm.com>
Subject: Re: [PATCH 3/3] perf report: Add weight[123] output fields

Hi Kan,

On Tue, Apr 9, 2024 at 9:37 AM Liang, Kan <kan.liang@...ux.intel.com> wrote:
>
>
>
> On 2024-04-08 8:06 p.m., Namhyung Kim wrote:
> > Add weight1, weight2 and weight3 fields to -F/--fields and their aliases
> > like 'ins_lat', 'p_stage_cyc' and 'retire_lat'.  Note that they are in
> > the sort keys too but the difference is that output fields will sum up
> > the weight values and display the average.
> >
> > In the sort key, users can see the distribution of weight value and I
> > think it's confusing we have local vs. global weight for the same weight.
> >
> > For example, I experiment with mem-loads events to get the weights.  On
> > my laptop, it seems only weight1 field is supported.
> >
> >   $ perf mem record -- perf test -w noploop
> >
> > Let's look at the noploop function only.  It has 7 samples.
> >
> >   $ perf script -F event,ip,sym,weight | grep noploop
> >   # event                         weight     ip           sym
> >   cpu/mem-loads,ldlat=30/P:           43     55b3c122bffc noploop
> >   cpu/mem-loads,ldlat=30/P:           48     55b3c122bffc noploop
> >   cpu/mem-loads,ldlat=30/P:           38     55b3c122bffc noploop    <--- same weight
> >   cpu/mem-loads,ldlat=30/P:           38     55b3c122bffc noploop    <--- same weight
> >   cpu/mem-loads,ldlat=30/P:           59     55b3c122bffc noploop
> >   cpu/mem-loads,ldlat=30/P:           33     55b3c122bffc noploop
> >   cpu/mem-loads,ldlat=30/P:           38     55b3c122bffc noploop    <--- same weight
> >
> > When you use the 'weight' sort key, it'd show entries with a separate
> > weight value separately.  Also note that the first entry has 3 samples
> > with weight value 38, so they are displayed together and the weight
> > value is the sum of 3 samples (114 = 38 * 3).
> >
> >   $ perf report -n -s +weight | grep -e Weight -e noploop
> >   # Overhead  Samples  Command   Shared Object   Symbol         Weight
> >        0.53%        3     perf   perf            [.] noploop    114
> >        0.18%        1     perf   perf            [.] noploop    59
> >        0.18%        1     perf   perf            [.] noploop    48
> >        0.18%        1     perf   perf            [.] noploop    43
> >        0.18%        1     perf   perf            [.] noploop    33
> >
> > If you use 'local_weight' sort key, you can see the actualy weight.
> >
> >   $ perf report -n -s +local_weight | grep -e Weight -e noploop
> >   # Overhead  Samples  Command   Shared Object   Symbol         Local Weight
> >        0.53%        3     perf   perf            [.] noploop    38
> >        0.18%        1     perf   perf            [.] noploop    59
> >        0.18%        1     perf   perf            [.] noploop    48
> >        0.18%        1     perf   perf            [.] noploop    43
> >        0.18%        1     perf   perf            [.] noploop    33
> >
> > But when you use the -F/--field option instead, you can see the average
> > weight for the while noploop funciton (as it won't group samples by
>
> %s/funciton/function/
>
> > weight value and use the default 'comm,dso,sym' sort keys).
> >
> >   $ perf report -n -F +weight | grep -e Weight -e noploop
> >   # Overhead  Samples  Weight1  Command  Shared Object  Symbol
> >        1.23%        7     42.4  perf     perf           [.] noploop
>
> I think the current +weight shows the sum of weight1 of all samples,
> (global weight). With this patch, it becomes an average (local_weight).
> The definition change may break the existing user script.
>
> Ideally, I think we should keep the meaning of the weight and
> local_weight as is.

Hmm.. then we may add 'avg_weight' or something.

But note that there's a subtle difference in the usage.  If you use
'weight' as a sort key (-s weight) it'd keep the existing behavior
that shows the sum (global_weight).  It'd show average only if
you use it as an output field (-F weight).

The issue of the sort key is that it cannot have the total sum
of weights for a function.  It'll have separate entries for each
weight for each function like in the above example.

Thanks,
Namhyung