Re: [Patch v5 0/6] Bug fixes on topdown events reordering

Ian Rogers
2024-10-02T14:31:22Z

On Tue, Oct 1, 2024 at 3:32 PM Ian Rogers <irogers@google.com> wrote:
>
> On Tue, Oct 1, 2024 at 2:02 PM Namhyung Kim <namhyung@kernel.org> wrote:
> >
> > On Fri, 13 Sep 2024 08:47:06 +0000, Dapeng Mi wrote:
> >
> > > Changes:
> > > v5 -> v6:
> > > * no function change.
> > > * rebase patchset to latest code of perf-tool-next tree.
> > > * Add Kan's reviewed-by tag.
> > >
> > > History:
> > > v4: https://lore.kernel.org/all/20240816122938.32228-1-dapeng1.mi@linux.intel.com/
> > > v3: https://lore.kernel.org/all/20240712170339.185824-1-dapeng1.mi@linux.intel.com/
> > > v2: https://lore.kernel.org/all/20240708144204.839486-1-dapeng1.mi@linux.intel.com/
> > > v1: https://lore.kernel.org/all/20240702224037.343958-1-dapeng1.mi@linux.intel.com/
> > >
> > > [...]
> >
> > Applied to perf-tools-next, thanks!
>
> I disagreed with an early patch set and the issue wasn't resolved. Specifically:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/commit/?h=perf-tools-next&id=3b5edc0421e2598a0ae7f0adcd592017f37e3cdf
> ```
> /* Followed by topdown events. */
> if (arch_is_topdown_metrics(lhs) && !arch_is_topdown_metrics(rhs))
> return -1;
> - if (!arch_is_topdown_metrics(lhs) && arch_is_topdown_metrics(rhs))
> + /*
> + * Move topdown events forward only when topdown events
> + * are not in same group with previous event.
> + */
> + if (!arch_is_topdown_metrics(lhs) && arch_is_topdown_metrics(rhs) &&
> + lhs->core.leader != rhs->core.leader)
> return 1;
> ```
> Is a broken comparator as the lhs then rhs behavior varies from the
> rhs then lhs behavior. The qsort implementation can randomly order the
> events.
> Please drop/revert.

Tihs is still in the tree:
https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/commit/?h=perf-tools-next&id=3b5edc0421e2598a0ae7f0adcd592017f37e3cdf
It should be invariant that for all comparators that:
a < b == b > a
This code breaks this property and so event sorting is broken.

Thanks,
Ian