Re: [PATCH v2 6/6] sched/deadline: Implement fallback mechanism for !fit case

[Pavan Kondeti <pkondeti@codeaurora.org>]

On Wed, Apr 29, 2020 at 07:39:50PM +0200, Dietmar Eggemann wrote:
> On 27/04/2020 16:17, luca abeni wrote:
> > Hi Juri,
> > 
> > On Mon, 27 Apr 2020 15:34:38 +0200
> > Juri Lelli <juri.lelli@redhat.com> wrote:
> > 
> >> Hi,
> >>
> >> On 27/04/20 10:37, Dietmar Eggemann wrote:
> >>> From: Luca Abeni <luca.abeni@santannapisa.it>
> >>>
> >>> When a task has a runtime that cannot be served within the
> >>> scheduling deadline by any of the idle CPU (later_mask) the task is
> >>> doomed to miss its deadline.
> >>>
> >>> This can happen since the SCHED_DEADLINE admission control
> >>> guarantees only bounded tardiness and not the hard respect of all
> >>> deadlines. In this case try to select the idle CPU with the largest
> >>> CPU capacity to minimize tardiness.
> >>>
> >>> Signed-off-by: Luca Abeni <luca.abeni@santannapisa.it>
> >>> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> > [...]
> >>> -		if (!cpumask_empty(later_mask))
> >>> -			return 1;
> >>> +		if (cpumask_empty(later_mask))
> >>> +			cpumask_set_cpu(max_cpu, later_mask);  
> >>
> >> Think we touched upon this during v1 review, but I'm (still?)
> >> wondering if we can do a little better, still considering only free
> >> cpus.
> >>
> >> Can't we get into a situation that some of the (once free) big cpus
> >> have been occupied by small tasks and now a big task enters the
> >> system and it only finds small cpus available, were it could have fit
> >> into bigs if small tasks were put onto small cpus?
> >>
> >> I.e., shouldn't we always try to best fit among free cpus?
> > 
> > Yes; there was an additional patch that tried schedule each task on the
> > slowest core where it can fit, to address this issue.
> > But I think it will go in a second round of patches.
> 
> Yes, we can run into this situation in DL, but also in CFS or RT.
> 
In CFS case, the misfit task handling in load balancer should help pulling
the BIG task running on the little CPUs. I get your point that we can run
into the same scenario with other scheduling class tasks.

> IMHO, this patch is aligned with the Capacity Awareness implementation
> in CFS and RT.
> 
> Capacity Awareness so far is 'find a CPU which fits the requirement of
> the task (Req)'. It's not (yet) find the best CPU.
> 
> CFS - select_idle_capacity() -> task_fits_capacity()
> 
>       Req: util(p) * 1.25 < capacity_of(cpu)
> 
> RT  - select_task_rq_rt(), cpupri_find_fitness() ->
>       rt_task_fits_capacity()
> 
>       Req: uclamp_eff_value(p) <= capacity_orig_of(cpu)
> 
> DL  - select_task_rq_dl(), cpudl_find() -> dl_task_fits_capacity()
> 
>       Req: dl_runtime(p)/dl_deadline(p) * 1024  <= capacity_orig_of(cpu)
> 
> 
> There has to be an "idle" (from the viewpoint of the task) CPU available
> with a fitting capacity. Otherwise a fallback mechanism applies.
> 
> CFS - best capacity handling in select_idle_capacity().
> 
> RT  - Non-fitting lowest mask
> 
> DL  - This patch
> 
> You did spot the rt-app 'delay' for the small tasks in the test case ;-)

Thanks for the hint. It was not clear to me why 1 msec delay is given for
the small tasks in the rt-app json description in the cover letter.
I get it now :-)

Thanks,
Pavan

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