Re: EEVDF and NUMA balancing

Julia Lawall
2024-03-10T09:39:54Z

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On Fri, 19 Jan 2024, Vincent Guittot wrote:

> On Thu, 18 Jan 2024 at 23:13, Julia Lawall <julia.lawall@inria.fr> 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@inria.fr> 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@inria.fr> 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.
> > >
> > > Regarding the trace above, do you know if anything happens on CPU22
> > > just before the scheduler tried to kick the ILB on it ?
> > >
> > > 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.
> >
> > I haven't figured out everything, but the attached graph shows
> > that TIF_POLLING_NRFLAG is not always set. Sometimes it is and sometimes
> > it isn't.
> >
> > In the graph, on core 57, the blue box and the green x are before and
> > after the call to cpuidle_idle_call(), resplectively. One can't see it in
> > this graph, but the green x comes before the blue box. So almost all of
> > the time, it is in cpuidle_idle_call(), only in the tiny gap between the x
> > and the box is it back in do_idle with TIF_POLLING_NRFLAG set.
> >
> > Afterwards, there is a diamond for the polling case and a triangle for the
> > non polling case. These also occur on clock ticks, and may be
> > microscopically closer to (polling) or further from (not polling) the
> > green x and blue box.
>
> Your problem really looks like weird timing.
>
> It would be good to know which idle states are selected ? or even
> better if it's possible, disable all but one idle state and see if one
> idle state in particular trigger your problem
>
> idle state can be disable here :
> echo 1 > /sys/devices/system/cpu/cpu*/cpuidle/state*/disable
>
> One possible sequence:
> tick is not stopped on the idle cpu
> tick fires on busy and idle cpus
> idle cpu wakes up and the wake up time varies depending of wakeup
> latency of the entered c-state
> busy cpu executes call_function_single_prep_ipi() and idle cpu could
> be already woken or not depending of the time to wake up
>
> >
> > I haven't yet studied what happens afterwards in the non polling case.
>
> Side point, according to your trace above, you can 2 consecutives real
> idle load balance so the patch that I proposed, should be able to
> trigger active migration because the nr_balance_failed will be != 0
> the 2nd idle load balance. Are I missing something ?

I have gotten access to a 2-socket ARM server:

Cavium ThunderX2 99xx (Vulcan), aarch64, 2 CPUs/node, 32 cores/CPU

Actually, I can observe the same behavior as on Intel. The issue is that
when one runs the benchmark with no configuration options, the threads
iteratively do some work and then synchronize on a barrier. To wait to
synchronize, they spin for 300K iterations and then sleep. On the Intel
6130, the 300K iterations is more than enough time for the other threads
to reach the barrier. On the above ARM machine, more time is required (or
the spinning takes less time), and the threads always end up sleeping.
It seems that this sleeping is sufficient to allow resolving any task
placement problem (I haven't looked into the details).

However, one can instruct the program to spin continuously and never sleep
before reaching a barrier, which replicates the Intel behavior on ARM. In
this case, we again get the big gaps on some cores and the highly variable
execution times.

OMP_WAIT_POLICY=active ./bt.B.x

Two graphs are attached. bt.B.x_pyxis-4_6.7.0_performance_4.pdf is
the worst case out of 20 runs with the default configuration. There are
many short gaps, with soms slightly longer ones in the lower right corner
(starting around 8 seconds). bt.B.spin_pyxis-4_6.7.0_performance_15.pdf
is the worst case out of 20 runs with spinning. There are much fewer
events in this graph so this graph includes the arrows for migrations,
small marks for wakeups, etc. There seem to be gaps in the upper right
continuously on some core from 7 seconds to the end of the execution.

julia

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