Re: [PATCH 0/8] sched/deadline: Return the best satisfying affinity and dl in cpudl_find

[Byungchul Park <byungchul.park@lge.com>]

On Tue, Mar 28, 2017 at 08:11:53AM +0100, Juri Lelli wrote:
> > > > For example:
> > > > 
> > > >    cpu 0 is running a task (dl: 10).
> > > >    cpu 1 is running a task (dl: 9).
> > > >    cpu 2 is running a task (dl: 8).
> > > >    cpu 3 is running a task (dl: 2).
> > > > 
> > > >    where cpu 3 want to push a task (affinity is 1 2 3 and dl is 1).
> > > 
> > > Hummm, but this should only happen if you disable admission control,
> > > right? Otherwise task's affinity can't be smaller that 0-3.
> > 
> > Hi Juri,
> > 
> > Can I ask you what is addmission control? Do you mean affinity setting?
> 
> sched_setattr() for DEADLINE tasks peforms a set of checks before
> admitting the task to the system. Please have a look at Documentation/
> scheduler/sched-deadline.txt::Section5 for what concerns affinity.

I see.

> > And do you mean s/disable/enable? Or am I misunderstanding?
> > 
> 
> No, I meant disable. The problem is that if you disable admission
> control the problem you are pointing out can happen, if admission
> control is enabled otherwise it can't, as we enforce that tasks have
> affinity equal to the root_domain span to which they belong. E.g, in
> your case the task will have affinity set to 0-3 (or it won't be able to
> enter the system), so that would make the problem go away.

I see.

> > > > In this case, the task should be migrated from cpu 3 to cpu 1, and
> > > > preempt cpu 1's task. However, current code just returns fail because
> > > > it fails at the affinity test with the maximum cpu, that is, cpu 0.
> > > > 
> > > > This patch set tries to find the best among ones satisfying task's
> > > > affinity and dl constraint until success or no more to see.
> > > > 
> > > 
> > > Anyway, do you have numbers showing how common is you fail scenario?
> > 
> > Actually, it very depends on how to set test environment. I can provide
> > you ones which generate many fails. IMHO, it's not a matter of frequency
> > but a matter of whether it works corrently. As you know, rt policy already
> > works corrently regarding this problem.
> > 
> 
> Right. But, my point is that if what you are highlighting turns out to
> be a pretty frequent situation, maybe we need to find a better data
> structure to speed up push operations or we will end up using the slow
> path most of the times, making the heap useless.

I totally agree with you. I will check it and let you know.

Thank you,
Byungchul