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

From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>, cgroups <cgroups@vger.kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	Valentin Schneider <valentin.schneider@arm.com>
Subject: Re: [regression] cpuset: offlined CPUs removed from affinity masks
Date: Wed, 19 Feb 2020 10:50:35 -0500 (EST)	[thread overview]
Message-ID: <59426509.702.1582127435733.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <20200219154740.GD698990@mtj.thefacebook.com>

----- On Feb 19, 2020, at 10:47 AM, Tejun Heo tj@kernel.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