RE: Scheduling tasks on idle cpu

[David Laight]

From: Qais Yousef
> Sent: 14 April 2022 00:51
> 
> On 04/12/22 08:39, David Laight wrote:
> > From: Qais Yousef
> > > Sent: 12 April 2022 00:35
> > >
> > > On 04/11/22 08:26, David Laight wrote:
> > > > From: Qais Yousef
> > > > > Sent: 09 April 2022 18:09
> > > > ...
> > > > > RT scheduler will push/pull tasks to ensure the task will get to run ASAP if
> > > > > there's another cpu at lower priority is available
> > > >
> > > > Does that actually happen?
> > >
> > > For RT tasks, yes. They should get distributed.
> >
> > Ok, that is something slightly different from what I'm seeing.
> 
> If you have multiple SCHED_FIFO/SCHED_RR tasks with the same priority, they
> don't end up being distributed on different CPUs? Assuming number of tasks is
> not higher than number of CPUs.
> 
> Generally if there are two RT tasks on the same CPU and there's another CPU
> that is running something that is lower priority than these two, then the lower
> priority of these 2 tasks should move to that CPU.
> 
> Eh, hope that's readable :-)

That is (just about) readable, and is happening.

> > > > I've seen the following:
> > > >   34533 [017]: sys_futex(uaddr: 1049104, op: 85, val: 1, utime: 1, uaddr2: 1049100, val3:
> 4000001)
> > > >   34533 [017]: sched_migrate_task: pid=34512 prio=120 orig_cpu=14 dest_cpu=17
> > > >   34533 [017]: sched_wakeup: pid=34512 prio=120 success=1 target_cpu=017
> > >
> > > prio=120 is a CFS task, no?
> >
> > CFS = 'normal time-slice processes ? Then yes.
> 
> Sorry, yes. CFS = SCHED_NORMAL/SCHED_OTHER.
> 
> >
> > > > and pid 34512 doesn't get scheduled until pid 34533 finally sleeps.
> > > > This is in spite of there being 5 idle cpu.
> > > > cpu 14 is busy running a RT thread, but migrating to cpu 17 seems wrong.
> > > >
> > > > This is on a RHEL7 kernel, I've not replicated it on anything recent.
> > > > But I've very much like a RT thread to be able to schedule a non-RT
> > > > thread to run on an idle cpu.
> > >
> > > Oh, you want CFS to avoid CPUs that are running RT tasks.
> > >
> > > We had a proposal in the past, but it wasn't good enough
> > >
> > > 	https://lore.kernel.org/lkml/1567048502-6064-1-git-send-email-jing-ting.wu@xxxxxxxxxxxx/
> >
> > That seems to be something different.
> > Related to something else I've seen where a RT process is scheduled
> > on its old cpu (to get the hot cache) but the process running on
> > that cpu is looping in kernel - so the RT process doesn't start.
> 
> I *think* you're hitting softirq latencies. Most likely it's the network RX
> softirq processing the packets. If this latency is a problem, then PREEMPT_RT
> [1] should help with this. For Android we hit this issue and there's a long
> living out of tree patch that I'm trying to find an upstream replacement for.

I suspect the costs of PREEMPT_RT would slow things down too much.
This test system has 40 cpu, 35 of them are RT and processing the same 'jobs'.
It doesn't really matter if one is delayed by the network irq + softirq code.
The problems arise if they all stop.
The 'job' list was protected by a mutex - usually not too bad.
But if a network irq interrupts the code while it holds the mutex then all
the RT tasks stall until the softirq code completes.
I've replaced the linked list with an array and used atomic_inc().

I can imagine that a PREEMPT_RT kernel will have the same problem
because (I think) all the spin locks get replaced by sleep locks.

> 
> There's a new knob to reduce how long netdev spends in the loop. Might be worth
> a try:
> 
> 	https://lore.kernel.org/netdev/1492619830-7561-1-git-send-email-tedheadster@xxxxxxxxx/
> 
> [1] https://wiki.linuxfoundation.org/realtime/start

I think the patch that runs the softirq in a separate thread might help.
But it probably needs a test to only to that if it would 'stall' a RT process.

> > I've avoided most of the pain that caused by not using a single
> > cv_broadcast() to wake up the 34 RT threads (in this config).
> > (Each kernel thread seemed to wake up the next one, so the
> > delays were cumulative.)
> > Instead there is a separate cv for each RT thread.
> > I actually want the 'herd of wildebeest' :-)
> 
> It seems you have a big RT app running in userspace. I thought initially you're
> hitting issues with random kthreads or something. If you have control over
> these tasks, then that should be easier to handle (as you suggest at the end).

I've a big app with a lot of RT threads doing network send/receive.
(All the packets as ~200 byte UDP, 50/sec on 1000+ port numbers.)
But there are other things going on as well.

> I'm not sure about the delays when using cv_broadcast(). Could it be the way
> this library is implemented is causing the problem rather than a kernel
> limitation?

I was definitely seeing the threads wake up one by one.
Every 10ms one of the RT threads wakes up and then wakes up all the others.
There weren't any 'extra' system calls, once one thread was running
in kernel the next one got woken up.
Most (and always) noticeable were the delays getting each cpu out
of its sleep state.
But if one of the required cpu was (eg) running the softint code
none of the latter ones would wake up.

> > > The approach in that patch modified RT to avoid CFS actually.
> >
> > Yes I want the CFS scheduler to pick an idle cpu in preference
> > to an active RT one.
> 
> I think that's what should happen. But I think it's racy. Vincent knows this
> code better though, so I'll defer to him.
> 
> >
> > > Can you verify whether the RT task woke up after task 34512 was migrated to CPU
> > > 17? Looking at the definition of available_idle_cpu() we should have avoided
> > > that CPU if the RT task was already running. Both waking up at the same time
> > > would explain what you see. Otherwise I'm not sure why it picked CPU 17.
> >
> > All 35 RT tasks are running when the request to schedule task 34512 is made.
> > (They wake every 10ms to process UDP/RTP audio packets.)
> > The RT task on cpu 17 carried on running until it ran out of work (after about 1ms).
> > Task 34512 then ran on cpu 17.
> >
> > In this case task 34512 actually finished quite quickly.
> > (It is creating and binding more UDP sockets.)
> > But it looks like if it were still running on the next 10ms 'tick'
> > it would be pre-empted by the RT task and be idle.
> > Not ideal when I'm trying to schedule a background activity.
> >
> > I don't think the load-balancer will ever pick it up.
> > All the process scheduling is happening far too fast.
> >
> > What I think might be happening is that the futex() code is requesting
> > the woken up thread run on the current cpu.
> 
> Hmm. Looking at kernel/futex/waitwake.c::futex_wake() it just ends up calling
> wake_up_process(). So that might not be the case.
> 
> > This can be advantageous in some circumstances - usually if you
> > know the current thread is about to sleep.
> > (I remember another scheduler doing that, but I can't remember why!
> > The only sequence I can think of is a shell doing fork+exec+wait.)
> > But it seems like a bad idea when a RT thread is waking a CFS one.
> > (Or any case where the one being woken is lower priority.)
> >
> > I might have to run the 'background tasks' at low RT priority
> > just to get them scheduled on idle cpu.
> 
> If you make it an RT task (which I think is a good idea), then the RT scheduler
> will handle it in the push/pull remark that seem to have started this
> discussion and get pushed/pulled to another CPU that is running lower priority
> task.

The problem is that while I'd like this thread to start immediately
what it is doing isn't THAT important.
There are other things that might run on the CFS scheduler that are
more important.
I can make it RT for experiments.

	David

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