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

[Peter Zijlstra]

On Tue, Aug 01, 2017 at 01:17:45PM -0700, Tejun Heo wrote:

> > What about the whole double accounting thing? Because currently cpuacct
> > and cpu do a fair bit of duplication. It would be very good to get rid
> > of that.
> 
> I'm not that sure at this point.  Here are my current thoughts on
> cpuacct.
> 
> * It is useful to have basic cpu statistics on cgroup without having
>   to enable the cpu controller, especially because enabling cpu
>   controller always changes how cpu cycles are distributed and
>   currently comes at some performance overhead.
> 
> * 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.

> * 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?