Re: EEVDF and NUMA balancing

From: Vincent Guittot
Date: Thu Jan 18 2024 - 12:11:17 EST

On Thu, 18 Jan 2024 at 17:50, Julia Lawall <julia.lawall@xxxxxxxx> wrote:
>
>
>
> On Thu, 18 Jan 2024, Vincent Guittot wrote:
>
> > Hi Julia,
> >
> > Sorry for the delay. I have been involved on other perf regression
> >
> > On Fri, 5 Jan 2024 at 18:27, Julia Lawall <julia.lawall@xxxxxxxx> wrote:
> > >
> > >
> > >
> > > On Fri, 5 Jan 2024, Julia Lawall wrote:
> > >
> > > >
> > > >
> > > > On Fri, 5 Jan 2024, Vincent Guittot wrote:
> > > >
> > > > > On Fri, 5 Jan 2024 at 15:51, Julia Lawall <julia.lawall@xxxxxxxx> wrote:
> > > > > >
> > > > > > > Your system is calling the polling mode and not the default
> > > > > > > cpuidle_idle_call() ? This could explain why I don't see such problem
> > > > > > > on my system which doesn't have polling
> > > > > > >
> > > > > > > Are you forcing the use of polling mode ?
> > > > > > > If yes, could you check that this problem disappears without forcing
> > > > > > > polling mode ?
> > > > > >
> > > > > > I expanded the code in do_idle to:
> > > > > >
> > > > > > if (cpu_idle_force_poll) { c1++;
> > > > > > tick_nohz_idle_restart_tick();
> > > > > > cpu_idle_poll();
> > > > > > } else if (tick_check_broadcast_expired()) { c2++;
> > > > > > tick_nohz_idle_restart_tick();
> > > > > > cpu_idle_poll();
> > > > > > } else { c3++;
> > > > > > cpuidle_idle_call();
> > > > > > }
> > > > > >
> > > > > > Later, I have:
> > > > > >
> > > > > > trace_printk("force poll: %d: c1: %d, c2: %d, c3: %d\n",cpu_idle_force_poll, c1, c2, c3);
> > > > > > flush_smp_call_function_queue();
> > > > > > schedule_idle();
> > > > > >
> > > > > > force poll, c1 and c2 are always 0, and c3 is always some non-zero value.
> > > > > > Sometimes small (often 1), and sometimes large (304 or 305).
> > > > > >
> > > > > > So I don't think it's calling cpu_idle_poll().
> > > > >
> > > > > I agree that something else
> > > > >
> > > > > >
> > > > > > x86 has TIF_POLLING_NRFLAG defined to be a non zero value, which I think
> > > > > > is sufficient to cause the issue.
> > > > >
> > > > > Could you trace trace_sched_wake_idle_without_ipi() ans csd traces as well ?
> > > > > I don't understand what set need_resched() in your case; having in
> > > > > mind that I don't see the problem on my Arm systems and IIRC Peter
> > > > > said that he didn't face the problem on his x86 system.
> > > >
> > > > TIF_POLLING_NRFLAG doesn't seem to be defined on Arm.
> > > >
> > > > Peter said that he didn't see the problem, but perhaps that was just
> > > > random. It requires a NUMA move to occur. I make 20 runs to be sure to
> > > > see the problem at least once. But another machine might behave
> > > > differently.
> > > >
> > > > I believe the call chain is:
> > > >
> > > > scheduler_tick
> > > > trigger_load_balance
> > > > nohz_balancer_kick
> > > > kick_ilb
> > > > smp_call_function_single_async
> > > > generic_exec_single
> > > > __smp_call_single_queue
> > > > send_call_function_single_ipi
> > > > call_function_single_prep_ipi
> > > > set_nr_if_polling <====== sets need_resched
> > > >
> > > > I'll make a trace to reverify that.
> > >
> > > This is what I see at a tick, which corresponds to the call chain shown
> > > above:
> > >
> > > bt.B.x-4184 [046] 466.410605: bputs: scheduler_tick: calling trigger_load_balance
> > > bt.B.x-4184 [046] 466.410605: bputs: trigger_load_balance: calling nohz_balancer_kick
> > > bt.B.x-4184 [046] 466.410605: bputs: trigger_load_balance: calling kick_ilb
> > > bt.B.x-4184 [046] 466.410607: bprint: trigger_load_balance: calling smp_call_function_single_async 22
> > > bt.B.x-4184 [046] 466.410607: bputs: smp_call_function_single_async: calling generic_exec_single
> > > bt.B.x-4184 [046] 466.410607: bputs: generic_exec_single: calling __smp_call_single_queue
> > > bt.B.x-4184 [046] 466.410608: bputs: __smp_call_single_queue: calling send_call_function_single_ipi
> > > bt.B.x-4184 [046] 466.410608: bputs: __smp_call_single_queue: calling call_function_single_prep_ipi
> > > bt.B.x-4184 [046] 466.410608: bputs: call_function_single_prep_ipi: calling set_nr_if_polling
> > > bt.B.x-4184 [046] 466.410609: sched_wake_idle_without_ipi: cpu=22
> >
> > I don't know if you have made progress on this in the meantime.
>
> Not really. Basically after do_idle, there is the call to
> flush_smp_call_function_queue that invokes the deposited functions, which
> in our case is at best going to raise a softirq, and the call to schedule.
> Raising a softirq doesn't happen because of the check for need_resched.
> But even if that test were removed, it would still not be useful because
> there would be the ksoftirqd running on the idle core that would eliminate
> the imbalance between the sockets. Maybe there could be some way to call
> run_rebalance_domains directly from nohz_csd_func, since
> run_rebalance_domains doesn't use its argument, but at the moment
> run_rebalance_domains is not visible to nohz_csd_func.

All this happen because we don't use an ipi, it should not use
ksoftirqd with ipi

>
> >
> > Regarding the trace above, do you know if anything happens on CPU22
> > just before the scheduler tried to kick the ILB on it ?
>
> I don't think so. It's idle.

Ok, so if it is idle for a while , I mean nothing happened on it, not
even spurious irq, It should have cleared its TIF_POLLING_NRFLAG

I would be good to trace the selected idle state

>
> > Have you found why TIF_POLLING_NRFLAG seems to be always set when the
> > kick_ilb happens ? It should be cleared once entering the idle state.
>
> Actually, I don't think it is always set. It switches back and forth
> between two cases. I will look for the traces that show that.
>
> > Could you check your cpuidle driver ?
>
> Check what specifically?

$ cat /sys/devices/system/cpu/cpuidle/current_driver
$ cat /sys/devices/system/cpu/cpuidle/current_governor

>
> thanks,
> julia