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

[Waiman Long]

On 7/28/22 16:44, Tejun Heo wrote:
> Hello,
>
> On Thu, Jul 28, 2022 at 03:21:26PM -0400, Waiman Long wrote:
> > On 7/28/22 15:02, Tejun Heo wrote:
> > > 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.
> Yeah, I hear you. I'm on the same page.
>
> > > 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.
> af
> So, the patch you proposed is making the code remember one special aspect of
> user requested configuration - whether it configured it or not, and trying
> to preserve that particular state as cpuset state changes. It addresses the
> immediate problem but it is a very partial approach. Let's say a task wanna
> be affined to one logical thread of each core and set its mask to 0x5555.
> Now, let's say cpuset got enabled and enforced 0xff and affined the task to
> 0xff. After a while, the cgroup got more cpus allocated and its cpuset now
> has 0xfff. Ideally, what should happen is the task now having the effective
> mask of 0x555. In practice, tho, it either would get 0xf55 or 0x55 depending
> on which way we decide to misbehave.

OK, I see what you want to accomplish. To fully address this issue, we 
will need to have a new cpumask variable in the the task structure which 
will be allocated if sched_setaffinity() is ever called. I can rework my 
patch to use this approach.

Thanks,
Longman