linux-kernel - Re: [regression] cpuset: offlined CPUs removed from affinity masks

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Message-ID: <20200219154740.GD698990@mtj.thefacebook.com>
Date:   Wed, 19 Feb 2020 10:47:40 -0500
From:   Tejun Heo <tj@...nel.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Li Zefan <lizefan@...wei.com>, cgroups <cgroups@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Valentin Schneider <valentin.schneider@....com>
Subject: Re: [regression] cpuset: offlined CPUs removed from affinity masks

On Wed, Feb 19, 2020 at 10:43:05AM -0500, Mathieu Desnoyers wrote:
> The regression I'm talking about here is that CONFIG_CPUSET=y changes the
> behavior of the sched_setaffinify system call, which existed prior to
> cpusets.
> 
> sched_setaffinity should behave in the same way for kernels configured with
> CONFIG_CPUSET=y or CONFIG_CPUSET=n.
> 
> The fact that cpuset decides to irreversibly change the task affinity mask
> may not be considered a regression if it has always done that, but changing
> the behavior of sched_setaffinity seems to fit the definition of a regression.

We generally use "regression" for breakages which weren't in past
versions but then appeared later. It has debugging implications
because if we know something is a regression, we generally can point
to the commit which introduced the bug either through examining the
history or bisection.

It is a silly bug, for sure, but slapping regression name on it just
confuses rather than helping anything.

-- 
tejun