Re: [PATCH v2 3/6] sched/deadline: Add dl_bw_capacity()

[Juri Lelli <juri.lelli@redhat.com>]

On 12/05/20 14:39, Dietmar Eggemann wrote:
> On 11/05/2020 10:01, Juri Lelli wrote:
> > On 06/05/20 17:09, Dietmar Eggemann wrote:
> >> On 06/05/2020 14:37, Juri Lelli wrote:
> >>> On 06/05/20 12:54, Dietmar Eggemann wrote:
> >>>> On 27/04/2020 10:37, Dietmar Eggemann wrote:
> 
> [...]
> 
> >>> to say that we actually want to check new tasks bw requirement against
> >>> the available bandwidth of the particular CPU they happen to be running
> >>> (and will continue to run) when setscheduler is called.
> >>
> >> By 'available bandwidth of the particular CPU' you refer to
> >> '\Sum_{cpu_rq(i)->rd->span} CPU capacity', right?
> > 
> > No. I was referring to the single CPU capacity. The capacity of the CPU
> > where a task is running when setscheduler is called for it (and DL AC
> > performed). See below, maybe more clear why I wondered about this case..
> 
> OK, got it! I was just confused since I don't think that this patch
> introduced the issue.
> 
> Before the patch 'int cpus = dl_bw_cpus(task_cpu(p))' was used which
> returns the number of cpus on the (default) rd (n). So for a single CPU
> (1024) we use n*1024.
> 
> I wonder if a fix for that should be part of this patch-set?

Not really, I guess. As you said, the issue was there already. We can
fix both situations with a subsequent patch. I just realized that we
have a problem by reviewing this set, but not this set job to fix it.

While you are at changing this part, it might be good to put a comment
(XXX fix this, or something) about the issue, so that we don't forget.

> [...]
> 
> >> ...
> >> [  144.920102] __dl_bw_capacity CPU3 rd->span=3-5 return 1338
> >> [  144.925607] sched_dl_overflow: [bash 1999] task_cpu(p)=3 cap=1338 cpus_ptr=3-5
> > 
> > So, here you are checking new task bw against 1338 which is 3*L
> > capacity. However, since load balance is disabled at this point for 3-5,
> > once admitted the task will only be able to run on CPU 3. Now, if more
> > tasks on CPU 3 are admitted the same way (up to 1338) I believe they
> > will start to experience deadline misses because only 446 will be
> > actually available to them until load balance is enabled below and they
> > are then free to migrate to CPUs 4 and 5.
> > 
> > Does it makes sense?
> 
> Yes, it does.
> 
> So my first idea was to only consider the CPU (i.e. its CPU capacity) in
> case we detect 'cpu_rq(cpu)->rd == def_root_domain'?
> 
> In case I re-enable load-balancing on cpuset '/', we can't make a task
> in cpuset 'B' DL since we hit this in __sched_setscheduler():
> 
> 4931           /*
> 4932            * Don't allow tasks with an affinity mask smaller than
> 4933            * the entire root_domain to become SCHED_DEADLINE.
> ...
> 4935            */
> 4936            if (!cpumask_subset(span, p->cpus_ptr) || ...
> 
> root@juno:~# echo 1 > /sys/fs/cgroup/cpuset/cpuset.sched_load_balance
> root@juno:~# echo $$ > /sys/fs/cgroup/cpuset/B/tasks
> root@juno:~# chrt -d --sched-runtime 8000 --sched-period 16000 -p 0 $$
> chrt: failed to set pid 2316's policy: Operation not permitted
> 
> So this task has to leave 'B' first I assume.

Right, because the span is back to contain all cpus (load balancing
enabled at root level), but tasks in 'B' still have affinity set to a
subset of them.