Re: [PATCH 1/2] cgroup/cpuset: Keep current cpus list if cpus affinity was explicitly set

[Waiman Long <longman@redhat.com>]

On 7/28/22 15:02, Tejun Heo wrote:
> Hello,
>
> On Thu, Jul 28, 2022 at 02:57:28PM -0400, Waiman Long wrote:
>> There can be a counter argument that if a user found out that there is not
>> enough cpus in a cpuset to meet its performance target, one can always
>> increase the number of cpus in the cpuset. Generalizing this behavior to all
>> the tasks irrespective if they have explicitly set cpus affinity before will
>> disallow this use case.
> This is nasty.

That is a nasty example, I know. There may be users depending on the 
existing behavior even if they don't know it. So I am a bit hesitant to 
change the default behavior like that. On the other hand, tasks that 
have explicitly set its cpu affinity certainly don't want to have 
unexpected change to that.

> The real solution here is separating out what user requested
> and the mask that cpuset (or cpu hotplug) needs to apply on top. ie.
> remember what the user requested in a separate cpumask and compute the
> intersection into p->cpus_maks whenever something changes and apply
> fallbacks on that final mask. Multiple parties updating the same variable is
> never gonna lead to anything consistent and we're patching up for whatever
> the immediate use case seems to need at the moment. That said, I'm not
> necessarily against patching it up but if you're interested in delving into
> it deeper, that'd be great.

I believe the current code is already restricting what cpu affinity that 
a user can request by limiting to those allowed by the current cpuset. 
Hotplug is another issue that may need to be addressed. I will update my 
patch to make it handle hotplug in a more graceful way.

Thanks,
Longman