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

[Tejun Heo]

Hello,

On Tue, Jan 11, 2022 at 11:29:50AM -0500, Daniel Jordan wrote:
...
> This problem arises with multithreaded jobs, but is also an issue in other
> places.  CPU activity from async memory reclaim (kswapd, cswapd?[5]) should be
> accounted to the cgroup that the memory belongs to, and similarly CPU activity
> from net rx should be accounted to the task groups that correspond to the
> packets being received.  There are also vague complaints from Android[6].

These are pretty big holes in CPU cycle accounting right now and I think
spend-first-and-backcharge is the right solution for most of them given
experiences from other controllers. That said,

> Each use case has its own requirements[7].  In padata and reclaim, the task
> group to account to is known ahead of time, but net rx has to spend cycles
> processing a packet before its destination task group is known, so any solution
> should be able to work without knowing the task group in advance.  Furthermore,
> the CPU controller shouldn't throttle reclaim or net rx in real time since both
> are doing high priority work.  These make approaches that run kthreads directly
> in a task group, like cgroup-aware workqueues[8] or a kernel path for
> CLONE_INTO_CGROUP, infeasible.  Running kthreads directly in cgroups also has a
> downside for padata because helpers' MAX_NICE priority is "shadowed" by the
> priority of the group entities they're running under.
> 
> The proposed solution of remote charging can accrue debt to a task group to be
> paid off or forgiven later, addressing all these issues.  A kthread calls the
> interface
> 
>     void cpu_cgroup_remote_begin(struct task_struct *p,
>                                  struct cgroup_subsys_state *css);
> 
> to begin remote charging to @css, causing @p's current sum_exec_runtime to be
> updated and saved.  The @css arg isn't required and can be removed later to
> facilitate the unknown cgroup case mentioned above.  Then the kthread calls
> another interface
> 
>     void cpu_cgroup_remote_charge(struct task_struct *p,
>                                   struct cgroup_subsys_state *css);
> 
> to account the sum_exec_runtime that @p has used since the first call.
> Internally, a new field cfs_bandwidth::debt is added to keep track of unpaid
> debt that's only used when the debt exceeds the quota in the current period.
> 
> Weight-based control isn't implemented for now since padata helpers run at
> MAX_NICE and so always yield to anything higher priority, meaning they would
> rarely compete with other task groups.

If we're gonna do this, let's please do it right and make weight based
control work too. Otherwise, its usefulness is pretty limited.

Thanks.

-- 
tejun