Re: periods and deadlines in SCHED_DEADLINE

From: Bjoern Brandenburg
Date: Wed Aug 04 2010 - 01:34:50 EST

On Aug 2, 2010, at 3:34 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Sun, 2010-07-11 at 09:32 +0200, Bjoern Brandenburg wrote:
>> Trying to infer whether a task is "hard" or "soft" from task
>> parameters is not a good idea, IMO. It's much better to make this an
>> explicit part of the task model that is configured via sched_setparam.
>> By default, tasks should be marked "soft" (this leaves more wiggle
>> room to the kernel); users who care can change the flag to "hard".
> I think we're in violent agreement here ;-) and I was convinced that was
> what we were talking about. The question was only how to represent that
> in the sched_param_ex structure, the options were:
> struct sched_param_ex params;
> params.flags |= SF_SOFT;
> sched_setscheduler_ex( .policy = SCHED_DEADLINE, .param = &params);
> vs
> sched_setscheduler_ex( .policy = SCHED_DEADLINE_{SOFT,HARD},
> .param = &params);

>From my point of view I don't really see a differenceâthe first approach probably lends itself better to adding additional variants but makes it harder to tweak a particular feature once it's used in userspace.

>> Taking a step back, I think the problem here is that we are trying to
>> shove too many concepts and properties into a single scheduler. Hard
>> (no tardiness) is different from soft (bounded tardiness) is different
>> from global is different from partitioned.
>> From my point of view, it makes no sense to support hard deadlines
>> under G-EDF (this is backed up by our schedulability studies [1]).
>> Hard deadlines are best served by a P-EDF implementation (that only
>> migrates on task creation/admission).
> The problem is more that we need to support things like cpu affinity and
> cpusets within the context of a 'global' scheduler.
> Using cpusets we can partition the 'load-balancer' and create clusters
> (root-domains in linux scheduler speak).
> Using cpu affinity we can limit tasks to a subset of their cluster's
> cpus.

Neither makes much sense in the context of a truly global scheduler. Creating non-overlapping clusters makes sense to create clustered schedulers.

> Esp. the latter is very hard to do, and I think we can get away with
> only allowing a single cpu or the full cluster (its a new policy, so
> there is no existing userspace to break).

It is probably a good idea to keep it as limited as possible in the beginning. There is no supporting theory afaik for overlapping clusters or non-uniform CPU affinities (i.e., nobody's looked at a case in which CPU affinities can vary from task to task).

> This ends up meaning we need to support both P-EDF and G-EDF for soft,
> and since we want to re-use pretty much all the code and only have a
> different admission test for hard (initially), it would end up also
> being P/G-EDF for hard

Couldn't you simply reject tasks that call setscheduler() with a combination of 'hard' and a CPU affinity mask with more than one CPU allowed?

> (even though as you rightly point out, hard G-EDF
> is pretty pointless -- but since the policy doesn't promise EDF, we
> could later improve it to be PD^2 or whatever, at which point global
> hard does start to make sense).
> (which I guess would suggest we use different policies instead of a
> flag, since that would make most sense if we end up replacing the hard
> part with another policy)

I don't think the implemented policy is just a hidden implementation detail that can be transparently changed in a future kernel version. Sure, other sporadic schedulers exist, but a lot of things in real-time systems depend on the scheduling policy (e.g., retry bounds for lock-free data structures, locking protocols, overhead accounting, etc.) so that changing it without revalidating/testing everything is not really an option. (At least not for applications that could be considered 'almost hard real-time').

> So what I want to have is a sporadic task scheduler, not an EDF
> scheduler (hence also the request to s/SCHED_EDF/SCHED_DEADLINE/ --
> avoiding the obvious SCHED_SPORADIC in order to avoid confusion with the
> POSIX thing).
> EDF is just the easiest of the many different ways to schedule a
> sporadic task set.

In theory I fully agree, but I strongly suspect that applications are going to assume a specific implementation anyway in practice. Changing the implemented scheduling policy for sporadic tasks would essentially force all existing embedded apps that make use of this API back into testing, which likely means that newer kernel versions could not be adopted for older projects (which may be the case anyway). In light of this, it may not be unreasonable to promise a specific policy.

However, a generic SCHED_DEADLINE does not preclude the option of supporting specific policies later, so this might not be a big deal in the end.

- BjÃrn--
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