Re: [RFC 15/16] sched/fair: Account kthread runtime debt for CFS bandwidth

[Peter Zijlstra]

On Wed, Jan 05, 2022 at 07:46:55PM -0500, Daniel Jordan wrote:
> As before, helpers in multithreaded jobs don't honor the main thread's
> CFS bandwidth limits, which could lead to the group exceeding its quota.
> 
> Fix it by having helpers remote charge their CPU time to the main
> thread's task group.  A helper calls a pair of new interfaces
> cpu_cgroup_remote_begin() and cpu_cgroup_remote_charge() (see function
> header comments) to achieve this.
> 
> This is just supposed to start a discussion, so it's pretty simple.
> Once a kthread has finished a remote charging period with
> cpu_cgroup_remote_charge(), its runtime is subtracted from the target
> task group's runtime (cfs_bandwidth::runtime) and any remainder is saved
> as debt (cfs_bandwidth::debt) to pay off in later periods.
> 
> Remote charging tasks aren't throttled when the group reaches its quota,
> and a task group doesn't run at all until its debt is completely paid,
> but these shortcomings can be addressed if the approach ends up being
> taken.
> 

*groan*... and not a single word on why it wouldn't be much better to
simply move the task into the relevant cgroup..