Re: EEVDF and NUMA balancing

Julia Lawall
2023-12-28T18:34:57Z

> > > > > > I'm surprised that you have mainly CPU_NEWLY_IDLE. Do you know the reason ?
> > > > >
> > > > > No. They come from do_idle calling the scheduler. I will look into why
> > > > > this happens so often.
> > > >
> > > > Hmm, the CPU was idle and received a need resched which triggered the
> > > > scheduler but there was nothing to schedule so it goes back to idle
> > > > after running a newly_idle _load_balance.
> > >
> > > I spent quite some time thinking the same until I saw the following code
> > > in do_idle:
> > >
> > > preempt_set_need_resched();
> > >
> > > So I have the impression that do_idle sets need resched itself.
> >
> > But of course that code is only executed if need_resched is true. But I
>
> Yes, that is your root cause. something, most probably in interrupt
> context, wakes up your CPU and expect to wake up a thread
>
> > don't know who would be setting need resched on each clock tick.
>
> that can be a timer, interrupt, ipi, rcu ...
> a trace should give you some hints

I have the impression that it is the goal of calling nohz_csd_func on each
clock tick that causes the calls to need_resched. If the idle process is
polling, call_function_single_prep_ipi just sets need_resched to get the
idle process to stop polling. But there is no actual task that the idle
process should schedule. The need_resched then prevents the idle process
from stealing, due to the CPU_NEWLY_IDLE flag, contradicting the whole
purpose of calling nohz_csd_func in the first place.

julia