Messages in this thread | | | Date | Fri, 21 Feb 2020 11:12:23 +0000 | From | Qais Yousef <> | Subject | Re: [PATCH 3/3] sched/rt: fix pushing unfit tasks to a better CPU |
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On 02/21/20 13:45, Pavan Kondeti wrote: > 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.
Oh, handy. Let me try this out.
I added it to my to-do to investigate it when I have time anyway.
In modern systems where L3 is spanning all CPUs, the migration isn't that costly, but it'd still be unnecessary wakeup latency that can add up.
Thanks
-- Qais Yousef
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