linux-kernel - Re: [PATCH] perf c2c: Fix sorting.

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Date:   Thu, 9 Jan 2020 22:46:11 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Andres Freund <andres@...razel.de>
Cc:     linux-kernel@...r.kernel.org, Jiri Olsa <jolsa@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Michael Petlan <mpetlan@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, stable@...r.kernel.org
Subject: Re: [PATCH] perf c2c: Fix sorting.

On Thu, Jan 09, 2020 at 09:00:41AM -0800, Andres Freund wrote:

SNIP

> > >  tools/perf/builtin-c2c.c | 10 ++++++----
> > >  1 file changed, 6 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/tools/perf/builtin-c2c.c b/tools/perf/builtin-c2c.c
> > > index e69f44941aad..f2e9d2b1b913 100644
> > > --- a/tools/perf/builtin-c2c.c
> > > +++ b/tools/perf/builtin-c2c.c
> > > @@ -595,8 +595,8 @@ tot_hitm_cmp(struct perf_hpp_fmt *fmt __maybe_unused,
> > >  {
> > >  	struct c2c_hist_entry *c2c_left;
> > >  	struct c2c_hist_entry *c2c_right;
> > > -	unsigned int tot_hitm_left;
> > > -	unsigned int tot_hitm_right;
> > > +	uint64_t tot_hitm_left;
> > > +	uint64_t tot_hitm_right;
> > 
> > that change looks right, but I can't see how that could
> > happened because of change in Fixes: tag
> > 
> > was the return statement of this function:
> > 
> >         return tot_hitm_left - tot_hitm_right;
> > 
> > considered to be 'unsigned int' and then converted to int64_t,
> > which would treat negative 'unsigned int' as big positive 'int64_t'?
> 
> Correct. So e.g. when comparing 1 and 2 tot_hitm, we'd get (int64_t)
> UINT_MAX as a result, which is obviously wrong. However, due to
> hist_entry__sort() returning int at the time, this was masked, as the
> int64_t was cast to int. Thereby again yielding a negative number for
> the comparisons of hist_entry__sort()'s result.  After
> hist_entry__sort() was fixed however, there never could be negative
> return values (but 0's are possible) of hist_entry__sort() for c2c.

I see.. ok

Acked-by: Jiri Olsa <jolsa@...hat.com>

thanks,
jirka