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

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Message-ID: <CAP-5=fX=5Ctgqr2pDodUrJpNfEcztBcRkCCTjhDEy+UeH9z1aA@mail.gmail.com>
Date: Wed, 2 Oct 2024 07:31:10 -0700
From: Ian Rogers <irogers@...gle.com>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Adrian Hunter <adrian.hunter@...el.com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Kan Liang <kan.liang@...ux.intel.com>, 
	Dapeng Mi <dapeng1.mi@...ux.intel.com>, linux-perf-users@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Yongwei Ma <yongwei.ma@...el.com>, 
	Dapeng Mi <dapeng1.mi@...el.com>
Subject: Re: [Patch v5 0/6] Bug fixes on topdown events reordering

On Tue, Oct 1, 2024 at 3:32 PM Ian Rogers <irogers@...gle.com> wrote:
>
> On Tue, Oct 1, 2024 at 2:02 PM Namhyung Kim <namhyung@...nel.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