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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

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

> Hello,
> 
> On Mon, Feb 17, 2020 at 11:03:07AM -0500, Mathieu Desnoyers wrote:
>> Hi,
>> 
>> Adding Tejun and the cgroups mailing list in CC for this cpuset regression I
>> reported last month.
>> 
>> Thanks,
>> 
>> Mathieu
>> 
>> ----- On Jan 16, 2020, at 12:41 PM, Mathieu Desnoyers
>> mathieu.desnoyers@efficios.com wrote:
>> 
>> > Hi,
>> > 
>> > I noticed the following regression with CONFIG_CPUSET=y. Note that
>> > I am not using cpusets at all (only using the root cpuset I'm given
>> > at boot), it's just configured in. I am currently working on a 5.2.5
>> > kernel. I am simply combining use of taskset(1) (setting the affinity
>> > mask of a process) and cpu hotplug. The result is that with
>> > CONFIG_CPUSET=y, setting the affinity mask including an offline CPU number
>> > don't keep that CPU in the affinity mask, and it is never put back when the
>> > CPU comes back online. CONFIG_CPUSET=n behaves as expected, and puts back
>> > the CPU into the affinity mask reported to user-space when it comes back
>> > online.
> 
> Because cpuset operations irreversibly change task affinity masks
> rather than masking them dynamically, the interaction has always been
> kinda broken. Hmm... Are there older kernel vesions which behave
> differently? Off the top of my head, I can't think of sth which could
> have changed that behavior recently but I could easily be missing
> something.

Hi Tejun,

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.

Thanks,

Mathieu


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