linux-kernel - Re: [PATCH] perf test: Make metric testing more robust.

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Date:   Wed, 4 Aug 2021 18:18:56 +0100
From:   John Garry <john.garry@...wei.com>
To:     Ian Rogers <irogers@...gle.com>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        "Namhyung Kim" <namhyung@...nel.org>,
        Kajol Jain <kjain@...ux.ibm.com>,
        linux-perf-users <linux-perf-users@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH] perf test: Make metric testing more robust.

On 04/08/2021 15:55, Ian Rogers wrote:
> 
> 
> On Wed, Aug 4, 2021, 2:11 AM John Garry <john.garry@...wei.com 
> <mailto:john.garry@...wei.com>> wrote:
> 
>     On 04/08/2021 08:25, Ian Rogers wrote:
>      > When testing metric expressions we fake counter values from 1 going
>      > upward. For some metrics this can yield negative values that are
>     clipped
>      > to zero, and then cause divide by zero failures. A workaround for
>     this
>      > case is to try a second time with counter values going in the
>     opposite
>      > direction.
>      >
>      > This case was seen in a metric like:
>      >    event1 / max(event2 - event3, 0)
> 
>     is this the standard method to make the metric evaluation fail when
>     results are not as expected? In this example, event2 should be greater
>     than event3 always. Dividing by max(x, 0) would seem a bit silly.
> 
> 
> I wouldn't say it was standard but it is in a metric a third party gave 
> us.

I agree that making it more robust is a good thing. But masking bogus 
expressions isn't great. After all, we're here to find them :)

 > It would be possible to get the same test failure on more standard
 > expressions, so it would be nice if these tests were more robust.

so something like this would fail also:
event1 / (event2 + event3 - 1 - event4)

assuming we have ascending values from 1 for event1. And this would seem 
a valid expression.

Anyway, it would be nice if we could reject max(0, x) and any divide by 
negative numbers, apart from your change.

Cheers,
john


>      >
>      > Signed-off-by: Ian Rogers <irogers@...gle.com
>     <mailto:irogers@...gle.com>>
>      > ---
>      >   tools/perf/tests/pmu-events.c | 32 ++++++++++++++++++++++++++------
>      >   1 file changed, 26 insertions(+), 6 deletions(-)
>      >
>      > diff --git a/tools/perf/tests/pmu-events.c
>     b/tools/perf/tests/pmu-events.c
>      > index b8aff8fb50d8..6c1cd58605c1 100644
>      > --- a/tools/perf/tests/pmu-events.c
>      > +++ b/tools/perf/tests/pmu-events.c
>      > @@ -600,8 +600,18 @@ static int test_parsing(void)
>      >                       }
>      >
>      >                       if (expr__parse(&result, &ctx,
>     pe->metric_expr, 0)) {
>      > -                             expr_failure("Parse failed", map, pe);
>      > -                             ret++;
>      > +                             /*
>      > +                              * Parsing failed, make numbers go
>     from large to
>      > +                              * small which can resolve divide
>     by zero
>      > +                              * issues.
>      > +                              */
>      > +                             k = 1024;
>      > +                             hashmap__for_each_entry((&ctx.ids),
>     cur, bkt)
>      > +                                     expr__add_id_val(&ctx,
>     strdup(cur->key), k--);
>      > +                             if (expr__parse(&result, &ctx,
>     pe->metric_expr, 0)) {
>      > +                                     expr_failure("Parse
>     failed", map, pe);
>      > +                                     ret++;
>      > +                             }
>      >                       }
>      >                       expr__ctx_clear(&ctx);
>      >               }
>      > @@ -656,10 +666,20 @@ static int metric_parse_fake(const char *str)
>      >               }
>      >       }
>      >
>      > -     if (expr__parse(&result, &ctx, str, 0))
>      > -             pr_err("expr__parse failed\n");
>      > -     else
>      > -             ret = 0;
>      > +     ret = 0;
>      > +     if (expr__parse(&result, &ctx, str, 0)) {
>      > +             /*
>      > +              * Parsing failed, make numbers go from large to
>     small which can
>      > +              * resolve divide by zero issues.
>      > +              */
>      > +             i = 1024;
>      > +             hashmap__for_each_entry((&ctx.ids), cur, bkt)
>      > +                     expr__add_id_val(&ctx, strdup(cur->key), i--);
>      > +             if (expr__parse(&result, &ctx, str, 0)) {
>      > +                     pr_err("expr__parse failed\n");
>      > +                     ret = -1;
>      > +             }
>      > +     }
>      >
>      >   out:
>      >       expr__ctx_clear(&ctx);
>      >
>