Re: [PATCH] perf c2c: Fix sorting.

[Andres Freund <andres@anarazel.de>]

Hi,

On 2020-01-09 09:48:22 +0100, Jiri Olsa wrote:
> On Wed, Jan 08, 2020 at 08:30:30PM -0800, Andres Freund wrote:
> > Commit 722ddfde366f ("perf tools: Fix time sorting") changed -
> > correctly so - hist_entry__sort to return int64. Unfortunately several
> > of the builtin-c2c.c comparison routines only happened to work due the
> > cast caused by the wrong return type.
> > 
> > This causes meaningless ordering of both the cacheline list, and the
> > cacheline details page. E.g a simple
> >   perf c2c record -a sleep 3
> >   perf c2c report
> > will result in cacheline table like
> >   =================================================
> >              Shared Data Cache Line Table
> >   =================================================
> >   #
> >   #        ----------- Cacheline ----------    Total      Tot  ----- LLC Load Hitm -----  ---- Store Reference ----  --- Load Dram ----      LLC    Total  ----- Core Load Hit -----  -- LLC Load Hit --
> >   # Index             Address  Node  PA cnt  records     Hitm    Total      Lcl      Rmt    Total    L1Hit   L1Miss       Lcl       Rmt  Ld Miss    Loads       FB       L1       L2       Llc       Rmt
> >   # .....  ..................  ....  ......  .......  .......  .......  .......  .......  .......  .......  .......  ........  ........  .......  .......  .......  .......  .......  ........  ........
> >   #
> >         0      0x7f0d27ffba00   N/A       0       52    0.12%       13        6        7       12       12        0         0         7       14       40        4       16        0         0         0
> >         1      0x7f0d27ff61c0   N/A       0     6353   14.04%     1475      801      674      779      779        0         0       718     1392     5574     1299     1967        0       115         0
> >         2      0x7f0d26d3ec80   N/A       0       71    0.15%       16        4       12       13       13        0         0        12       24       58        1       20        0         9         0
> >         3      0x7f0d26d3ec00   N/A       0       98    0.22%       23       17        6       19       19        0         0         6       12       79        0       40        0        10         0
> > i.e. with the list not being ordered by Total Hitm.
> > 
> > Fixes: 722ddfde366f ("perf tools: Fix time sorting")
> > Signed-off-by: Andres Freund <andres@anarazel.de>
> > Cc: Jiri Olsa <jolsa@kernel.org>
> > Cc: Andi Kleen <ak@linux.intel.com>
> > Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> > Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> > Cc: Michael Petlan <mpetlan@redhat.com>
> > Cc: Namhyung Kim <namhyung@kernel.org>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: stable@vger.kernel.org # v3.16+
> > ---
> >  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 briefly looked for places outside of c2c for similar issues in
hist_entry comparison routines, but didn't find any.

Greetings,

Andres Freund