RE: Scheduling tasks on idle cpu

[David Laight <David.Laight@ACULAB.COM>]

From: Steven Rostedt
> Sent: 11 April 2022 10:27
> 
> On Mon, 11 Apr 2022 08:26:33 +0000
> David Laight <David.Laight@ACULAB.COM> wrote:
> 
> > Does that actually happen?
> > 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
> > and pid 34512 doesn't get scheduled until pid 34533 finally sleeps.
> > This is in spite of there being 5 idle cpu.
> 
> What's the topology? I believe the scheduler will refrain from
> migrating tasks to idle CPUs that are on other NUMA nodes as much as
> possible. Were those other 5 idle CPUs on another node?

There are two physical cpu with 20 cores each (with hyperthreading).
16, 18, 34, 36 and 38 were idle.
So both 16 and 18 should be on the same NUMA node.
All the others are running the same RT thread code.

	David

> 
> -- Steve
> 
> 
> > 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.

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)