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

[Daniel Jordan]

On Wed, Jan 12, 2022 at 10:18:16AM -1000, Tejun Heo wrote:
> Hello,

Hi, Tejun.

> 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.

Ok, understood.

Doing it as presented is an incremental step and all that's required for
this.  I figured weight could be added later with the first user that
actually needs it.

I did prototype weight too, though, just to see if it was all gonna work
together, so given how the discussion elsewhere in the thread is going,
I might respin the scheduler part of this with another use case and
weight-based control included.

I got this far, do the interface and CFS skeleton seem sane?  Both are
basically unchanged with weight-based control included, the weight parts
are just more code on top.

Thanks for looking.