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

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Message-ID: <59426509.702.1582127435733.JavaMail.zimbra@efficios.com>
Date:   Wed, 19 Feb 2020 10:50:35 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Tejun Heo <tj@...nel.org>
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 Feb 19, 2020, at 10:47 AM, Tejun Heo tj@...nel.org wrote:

> 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.

I can look into figuring out the commit introducing this issue, which I
suspect will be close to the introduction of CONFIG_CPUSET into the
kernel (which was ages ago). I'll check and let you know.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com