Messages in this thread | | | From | Vincent Guittot <> | Date | Thu, 5 Sep 2019 16:01:51 +0200 | Subject | Re: [PATCH 1/1] sched/rt: avoid contend with CFS task |
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Hi Jing-Ting,
On Thu, 5 Sep 2019 at 15:26, Jing-Ting Wu <jing-ting.wu@mediatek.com> wrote: > > On Fri, 2019-08-30 at 15:55 +0100, Qais Yousef wrote: > > On 08/29/19 11:38, Valentin Schneider wrote: > > > On 29/08/2019 04:15, Jing-Ting Wu wrote: > > > > At original linux design, RT & CFS scheduler are independent. > > > > Current RT task placement policy will select the first cpu in > > > > lowest_mask, even if the first CPU is running a CFS task. > > > > This may put RT task to a running cpu and let CFS task runnable. > > > > > > > > So we select idle cpu in lowest_mask first to avoid preempting > > > > CFS task. > > > > > > > > > > Regarding the RT & CFS thing, that's working as intended. RT is a whole > > > class above CFS, it shouldn't have to worry about CFS. > > > > > > On the other side of things, CFS does worry about RT. We have the concept > > > of RT-pressure in the CFS scheduler, where RT tasks will reduce a CPU's > > > capacity (see fair.c::scale_rt_capacity()). > > > > > > CPU capacity is looked at on CFS wakeup (see wake_cap() and > > > find_idlest_cpu()), and the periodic load balancer tries to spread load > > > over capacity, so it'll tend to put less things on CPUs that are also > > > running RT tasks. > > > > > > If RT were to start avoiding rqs with CFS tasks, we'd end up with a nasty > > > situation were both are avoiding each other. It's even more striking when > > > you see that RT pressure is done with a rq-wide RT util_avg, which > > > *doesn't* get migrated when a RT task migrates. So if you decide to move > > > a RT task to an idle CPU "B" because CPU "A" had runnable CFS tasks, the > > > CFS scheduler will keep seeing CPU "B" as not significantly RT-pressured > > > while that util_avg signal ramps up, whereas it would correctly see CPU > > > "A" as RT-pressured if the RT task previously ran there. > > > > > > So overall I think this is the wrong approach. > > > > I like the idea, but yeah tend to agree the current approach might not be > > enough. > > > > I think the major problem here is that on generic systems where CFS is a first > > class citizen, RT tasks can be hostile to them - not always necessarily for a > > good reason. > > > > To further complicate the matter, even among CFS tasks we can't tell which are > > more important than the others - though hopefully latency-nice proposal will > > make the situation better. > > > > So I agree we have a problem here, but I think this patch is just a temporary > > band aid and we need to do better. Though I have no concrete suggestion yet on > > how to do that. > > > > Another thing I couldn't quantify yet how common and how severe this problem is > > yet. Jing-Ting, if you can share the details of your use case that'd be great. > > > > Cheers > > > > -- > > Qais Yousef > > > I agree that the nasty situation will happen.The current approach and this patch might not be enough.
RT task should not harm its cache hotness and responsiveness for the benefit of a CFS task
> But for requirement of performance, I think it is better to differentiate between idle CPU and CPU has CFS task. > > For example, we use rt-app to evaluate runnable time on non-patched environment. > There are (NR_CPUS-1) heavy CFS tasks and 1 RT Task. When a CFS task is running, the RT task wakes up and choose the same CPU. > The CFS task will be preempted and keep runnable until it is migrated to another cpu by load balance. > But load balance is not triggered immediately, it will be triggered until timer tick hits with some condition satisfied(ex. rq->next_balance).
Yes you will have to wait for the next tick that will trigger an idle load balance because you have an idle cpu and 2 runnable tack (1 RT + 1CFS) on the same CPU. But you should not wait for more than 1 tick
The current load_balance doesn't handle correctly the situation of 1 CFS and 1 RT task on same CPU while 1 CPU is idle. There is a rework of the load_balance that is under review on the mailing list that fixes this problem and your CFS task should migrate to the idle CPU faster than now
> CFS tasks may be runnable for a long time. In this test case, it increase 332.091 ms runnable time for CFS task. > > The detailed log is shown as following, CFS task(thread1-6580) is preempted by RT task(thread0-6674) about 332ms:
332ms is quite long and is probably not an idle load blanace but a busy load balance
> thread1-6580 [003] dnh2 94.452898: sched_wakeup: comm=thread0 pid=6674 prio=89 target_cpu=003 > thread1-6580 [003] d..2 94.452916: sched_switch: prev_comm=thread1 prev_pid=6580 prev_prio=120 prev_state=R ==> next_comm=thread0 next_pid=6674 next_prio=89 > .... 332.091ms > krtatm-1930 [001] d..2 94.785007: sched_migrate_task: comm=thread1 pid=6580 prio=120 orig_cpu=3 dest_cpu=1 > krtatm-1930 [001] d..2 94.785020: sched_switch: prev_comm=krtatm prev_pid=1930 prev_prio=100 prev_state=S ==> next_comm=thread1 next_pid=6580 next_prio=120
your CFS task has not moved on the idle CPU but has replaced another task
Regards, Vincent > > So I think choose idle CPU at RT wake up flow could reduce the CFS runnable time. > > > Best regards, > Jing-Ting Wu > >
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