Re: [PATCH 2/2] sched: Implement interface for cgroup unified hierarchy

[Tejun Heo <tj@kernel.org>]

Hello, Peter.

On Tue, Aug 01, 2017 at 11:40:38PM +0200, Peter Zijlstra wrote:
> > * On cgroup2, there is only one hierarchy.  It'd be great to have
> >   basic resource accounting enabled by default on all cgroups.  Note
> >   that we couldn't do that on v1 because there could be any number of
> >   hierarchies and the cost would increase with the number of
> >   hierarchies.
> 
> Yes, the whole single hierarchy thing makes doing away with the double
> accounting possible.

Yeah, we can either do that or make it cheaper so that we can have
basic stats by default.

> > * It is bothersome that we're walking up the tree each time for
> >   cpuacct although being percpu && just walking up the tree makes it
> >   relatively cheap.
> 
> So even if its only CPU local accounting, you still have all the pointer
> chasing and misses, not to mention that a faster O(depth) is still
> O(depth).
> 
> >		Anyways, I'm thinking about shifting the
> >   aggregation to the reader side so that the hot path always only
> >   updates local counters in a way which can scale even when there are
> >   a lot of (idle) cgroups.  Will follow up on this later.
> 
> Not entirely sure I follow, we currently only update the current cgroup
> and its immediate parents, no? Or are you looking to only account into
> the current cgroup and propagate into the parents on reading?

Yeah, shifting the cost to the readers and being smart with
propagation so that reading isn't O(nr_descendants) but
O(nr_descendants_which_have_run_since_last_read).  That way, we can
show the basic stats without taxing the hot paths with reasonable
scalability.

I have a couple questions about cpuacct tho.

* The stat file is sampling based and the usage files are calculated
  from actual scheduling events.  Is this because the latter is more
  accurate?

* Why do we have user/sys breakdown in usage numbers?  It tries to
  distinguish user or sys by looking at task_pt_regs().  I can't see
  how this would work (e.g. interrupt handlers never schedule) and w/o
  kernel preemption, the sys part is always zero.  What is this number
  supposed to mean?

Thanks.

-- 
tejun