Re: [PATCH v10 2/5] sched: CGroup tagging interface for core scheduling

[Peter Zijlstra]

On Fri, Jan 22, 2021 at 08:17:01PM -0500, Joel Fernandes (Google) wrote:
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

I'm thinking this is too much credit, I didn't write much of this.

> Marks all tasks in a cgroup as matching for core-scheduling.
> 
> A task will need to be moved into the core scheduler queue when the cgroup
> it belongs to is tagged to run with core scheduling.  Similarly the task
> will need to be moved out of the core scheduler queue when the cgroup
> is untagged.
> 
> Also after we forked a task, its core scheduler queue's presence will
> need to be updated according to its new cgroup's status.
> 
> Use stop machine mechanism to update all tasks in a cgroup to prevent a
> new task from sneaking into the cgroup, and missed out from the update
> while we iterates through all the tasks in the cgroup.  A more complicated
> scheme could probably avoid the stop machine.  Such scheme will also
> need to resovle inconsistency between a task's cgroup core scheduling
> tag and residency in core scheduler queue.
> 
> We are opting for the simple stop machine mechanism for now that avoids
> such complications.
> 
> Core scheduler has extra overhead.  Enable it only for core with
> more than one SMT hardware threads.

Very little actual words on the desired and implemented semantics of the
interface, while the patch contains much non-obvious complication.