Messages in this thread | | | Date | Thu, 18 Jan 2024 23:13:52 +0100 (CET) | From | Julia Lawall <> | Subject | Re: EEVDF and NUMA balancing |
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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.
I haven't yet studied what happens afterwards in the non polling case.
julia
> > Could you check your cpuidle driver ? > > Vincent > > > > > julia >[unhandled content-type:application/pdf]
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