Re: [PATCH 3/3] sched/rt: fix pushing unfit tasks to a better CPU

[Pavan Kondeti]

On Wed, Feb 19, 2020 at 02:02:44PM +0000, Qais Yousef wrote:
> On 02/17/20 13:53, Qais Yousef wrote:
> > On 02/17/20 14:53, Pavan Kondeti wrote:
> > > I notice a case where tasks would migrate for no reason (happens without this
> > > patch also). Assuming BIG cores are busy with other RT tasks. Now this RT
> > > task can go to *any* little CPU. There is no bias towards its previous CPU.
> > > I don't know if it makes any difference but I see RT task placement is too
> > > keen on reducing the migrations unless it is absolutely needed.
> > 
> > In find_lowest_rq() there's a check if the task_cpu(p) is in the lowest_mask
> > and prefer it if it is.
> > 
> > But yeah I see it happening too
> > 
> > https://imgur.com/a/FYqLIko
> > 
> > Tasks on CPU 0 and 3 swap. Note that my tasks are periodic but the plots don't
> > show that.
> > 
> > I shouldn't have changed something to affect this bias. Do you think it's
> > something I introduced?
> > 
> > It's something maybe worth digging into though. I'll try to have a look.
> 
> FWIW, I dug a bit into this and I found out we have a thundering herd issue.
> 
> Since I just have a set of periodic task that all start together,
> select_task_rq_rt() ends up selecting the same fitting CPU for all of them
> (CPU1). The end up all waking up on CPU1, only to get pushed back out
> again with only one surviving.
> 
> This reshuffles the task placement ending with some tasks being swapped.
> 
> I don't think this problem is specific to my change and could happen without
> it.
> 
> The problem is caused by the way find_lowest_rq() selects a cpu in the mask
> 
> 1750                         best_cpu = cpumask_first_and(lowest_mask,
> 1751                                                      sched_domain_span(sd));
> 1752                         if (best_cpu < nr_cpu_ids) {
> 1753                                 rcu_read_unlock();
> 1754                                 return best_cpu;
> 1755                         }
> 
> It always returns the first CPU in the mask. Or the mask could only contain
> a single CPU too. The end result is that we most likely end up herding all the
> tasks that wake up simultaneously to the same CPU.
> 
> I'm not sure how to fix this problem yet.
> 

Yes, I have seen this problem too. This is not limited to RT even fair class
(find_energy_efficient_cpu path) also have the same issue. There is a window
where we select a CPU for the task and the task being queued there. Because of
this, we may select the same CPU for two successive waking tasks. Turning off
TTWU_QUEUE sched feature addresses this up to some extent. At least it would
solve the cases like multiple tasks getting woken up from an interrupt handler.

Thanks,
Pavan

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