linux-kernel - Re: [PATCH] perf tests switch-tracking: Fix timestamp comparison

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Message-ID: <CAP-5=fXSaYcYp4YVvD8aRXNDuCN5f1QM6KkbNF4Sr+paTFkevQ@mail.gmail.com>
Date: Fri, 23 May 2025 09:52:44 -0700
From: Ian Rogers <irogers@...gle.com>
To: Leo Yan <leo.yan@....com>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Adrian Hunter <adrian.hunter@...el.com>, "Liang, Kan" <kan.liang@...ux.intel.com>, 
	James Clark <james.clark@...aro.org>, linux-perf-users@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf tests switch-tracking: Fix timestamp comparison

On Fri, May 23, 2025 at 1:10 AM Leo Yan <leo.yan@....com> wrote:
>
> On Thu, May 22, 2025 at 10:57:41PM -0300, Arnaldo Carvalho de Melo wrote:
> > On Thu, May 22, 2025 at 10:55:46PM -0300, Arnaldo Carvalho de Melo wrote:
> > > On Mon, Mar 31, 2025 at 06:27:59PM +0100, Leo Yan wrote:
> > > > The test might fail on the Arm64 platform with the error:
> > > >
> > > >   perf test -vvv "Track with sched_switch"
> > > >   Missing sched_switch events
> > > >
> > > > The issue is caused by incorrect handling of timestamp comparisons. The
> > > > comparison result, a signed 64-bit value, was being directly cast to an
> > > > int, leading to incorrect sorting for sched events.
> > > >
> > > > Fix this by explicitly returning 0, 1, or -1 based on whether the result
> > > > is zero, positive, or negative.
> > > >
> > > > Fixes: d44bc5582972 ("perf tests: Add a test for tracking with sched_switch")
> > > > Signed-off-by: Leo Yan <leo.yan@....com>
> > >
> > > How can I reproduce this?
> > >
> > > Testing on a rpi5, 64-bit debian, this test passes:
>
> Sorry that I did not give precise info for reproducing the failure.
> The case does not fail everytime, usually I can trigger the failure
> after run 20 ~ 30 times:
>
> # while true; do perf test "Track with sched_switch"; done
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : FAILED!
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : FAILED!
> 106: Track with sched_switch                                         : Ok
> 106: Track with sched_switch                                         : Ok

Just to advertise the "-r" or "--runs-per-test" option:
```
$ perf test -v -r 100 "Track with sched_switch"
```
The test is exclusive so you won't get any parallelism from this, but
you could in other cases.

Thanks,
Ian

> I used cross compiler to build Perf tool on my host machine and tested on
> Debian / Juno board.  Generally, I think this issue is not very specific
> to GCC versions.  As both internal CI and my local env can reproduce the
> issue.
>
> Please let me know if need any more info.  Thanks!
>
>
> ---8<---
>
> My Host Build compiler:
>
> # aarch64-linux-gnu-gcc --version
> aarch64-linux-gnu-gcc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
>
> Juno Board:
>
> # lsb_release -a
> No LSB modules are available.
> Distributor ID: Debian
> Description:    Debian GNU/Linux 12 (bookworm)
> Release:        12
> Codename:       bookworm