Re: [PATCH 1/2] sched/deadline: add per rq tracking of admitted bandwidth

[Peter Zijlstra]

On Fri, Feb 12, 2016 at 06:05:30PM +0100, Peter Zijlstra wrote:
> Having two separate means of accounting this also feels more fragile
> than one would want.
> 
> Let me think a bit about this.

I think there's a fundamental problem that makes the whole notion of
per-rq accounting 'impossible'.

On hot-unplug we only migrate runnable tasks, all blocked tasks remain
on the dead cpu. This would very much include their bandwidth
requirements.

This means that between a hot-unplug and the moment that _all_ those
blocked tasks have ran at least once, the sum of online bandwidth
doesn't match and we can get into admission trouble (same for GRUB,
which can also use per-rq bw like this).

The main problem is that there is no real way to find blocked tasks;
currently the only way is to iterate _all_ tasks and filter on
task_cpu().

We could of course add a blocked tree/list for deadline tasks, to
explicitly keep track of all these; this would allow migrating blocked
tasks on hotplug and avoid the real ugly I think. But I've not tried
yet.