Messages in this thread | | | Date | Mon, 17 Feb 2020 13:53:07 +0000 | From | Qais Yousef <> | Subject | Re: [PATCH 3/3] sched/rt: fix pushing unfit tasks to a better CPU |
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On 02/17/20 14:53, Pavan Kondeti wrote: > Hi Qais, > > On Fri, Feb 14, 2020 at 04:39:49PM +0000, Qais Yousef wrote: > > [...] > > > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c > > index 0c8bac134d3a..5ea235f2cfe8 100644 > > --- a/kernel/sched/rt.c > > +++ b/kernel/sched/rt.c > > @@ -1430,7 +1430,7 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags) > > { > > struct task_struct *curr; > > struct rq *rq; > > - bool test; > > + bool test, fit; > > > > /* For anything but wake ups, just return the task_cpu */ > > if (sd_flag != SD_BALANCE_WAKE && sd_flag != SD_BALANCE_FORK) > > @@ -1471,16 +1471,32 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags) > > unlikely(rt_task(curr)) && > > (curr->nr_cpus_allowed < 2 || curr->prio <= p->prio); > > > > - if (test || !rt_task_fits_capacity(p, cpu)) { > > + fit = rt_task_fits_capacity(p, cpu); > > + > > + if (test || !fit) { > > int target = find_lowest_rq(p); > > > > - /* > > - * Don't bother moving it if the destination CPU is > > - * not running a lower priority task. > > - */ > > - if (target != -1 && > > - p->prio < cpu_rq(target)->rt.highest_prio.curr) > > - cpu = target; > > + if (target != -1) { > > + /* > > + * Don't bother moving it if the destination CPU is > > + * not running a lower priority task. > > + */ > > + if (p->prio < cpu_rq(target)->rt.highest_prio.curr) { > > + > > + cpu = target; > > + > > + } else if (p->prio == cpu_rq(target)->rt.highest_prio.curr) { > > + > > + /* > > + * If the priority is the same and the new CPU > > + * is a better fit, then move, otherwise don't > > + * bother here either. > > + */ > > + fit = rt_task_fits_capacity(p, target); > > + if (fit) > > + cpu = target; > > + } > > + } > > I understand that we are opting for the migration when priorities are tied but > the task can fit on the new task. But there is no guarantee that this task > stay there. Because any CPU that drops RT prio can pull the task. Then why > not leave it to the balancer?
This patch does help in the 2 RT task test case. Without it I can see a big delay for the task to migrate from a little CPU to a big one, although the big is free.
Maybe my test is too short (1 second). The delay I've seen is 0.5-0.7s..
https://imgur.com/a/qKJk4w4
Maybe I missed the real root cause. Let me dig more.
> > 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.
Thanks
-- Qais Yousef
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