Re: [PATCH v4 15/19] sched: Fix migrate_disable() vs rt/dl balancing

[Qais Yousef]

On 03/05/21 15:41, Valentin Schneider wrote:
> On 05/03/21 15:56, Peter Zijlstra wrote:
> > On Sat, Dec 26, 2020 at 01:54:45PM +0000, Qais Yousef wrote:
> >>
> >> > +static inline struct task_struct *get_push_task(struct rq *rq)
> >> > +{
> >> > +	struct task_struct *p = rq->curr;
> >>
> >> Shouldn't we verify the class of the task here? The RT task in migration
> >> disabled could have been preempted by a dl or stopper task. Similarly, the dl
> >> task could have been preempted by a stopper task.
> >>
> >> I don't think an RT task should be allowed to push a dl task under any
> >> circumstances?
> >
> > Hmm, quite. Fancy doing a patch?
> 
> Last time we talked about this, I looked into
> 
>   push_rt_task() + find_lowest_rq()
> 
> IIRC, with how
> 
>   find_lowest_rq() + cpupri_find_fitness()
> 
> currently work, find_lowest_rq() should return -1 in push_rt_task() if
> rq->curr is DL (CPUPRI_INVALID). IOW, Migration-Disabled RT tasks shouldn't
> actually interfere with DL tasks (unless a DL task gets scheduled after we
> drop the rq lock and kick the stopper, but we have that problem everywhere
> including CFS active balance).

This makes it less of a problem true, but AFAICT this can still happen in the
pull path.

Anyways, here's the patch as extra bolts and braces to be considered.

Thanks

--
Qais Yousef

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