linux-kernel - Re: [PATCH] sched/idle: Prevent stopping the tick when there is no cpuidle driver

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Date: Wed, 10 Jan 2024 11:20:01 +0100
From: Pierre Gondois <pierre.gondois@....com>
To: Anna-Maria Behnsen <anna-maria@...utronix.de>,
 linux-kernel@...r.kernel.org
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, Daniel Bristot de Oliveira
 <bristot@...hat.com>, Valentin Schneider <vschneid@...hat.com>,
 Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched/idle: Prevent stopping the tick when there is no
 cpuidle driver

Hello Anna-Maria,

On 1/9/24 17:24, Anna-Maria Behnsen wrote:
> Hello Pierre,
> 
> Pierre Gondois <pierre.gondois@....com> writes:
> 
>> Hello Anna-Maria,
>>
>> On 12/15/23 14:05, Anna-Maria Behnsen wrote:
>>> When there is no cpuidle driver, the system tries to stop the tick even if
>>> the system is fully loaded. But stopping the tick is not for free and it
>>> decreases performance on a fully loaded system. As there is no (cpuidle)
>>> framework which brings CPU in a power saving state when nothing needs to be
>>> done, there is also no power saving benefit when stopping the tick.
>>
>> Just in case is wasn't taken into consideration:
>> -
>> Stopping the tick isn't free on a busy system, but it should also cost
>> something to regularly handle ticks on each CPU of an idle system.
>>
>> FWIU, disabling the ticks also allows to add a CPU to the 'nohz.idle_cpus_mask'
>> mask, which helps the idle load balancer picking an idle CPU to do load
>> balancing for all the idle CPUs (cf. kick_ilb()).
>>
>> It seems better to do one periodic balancing for all the idle CPUs rather
>> than periodically waking-up all CPUs to try to balance.
>>
>> -
>> I would have assumed that if the system was fully loaded, ticks would
>> not be stopped, or maybe I misunderstood the case.
>> I assume the wake-up latency would be improved if the tick doesn't
>> have to be re-setup again.
>>
> 
> Your answer confuses me a little...
> 
> When there is a cpuidle driver, trying to stop the tick is not done
> unconditionally. It is only done when the CPU is in a state that it
> could go into a deeper C sleep - this is decided by cpuidle
> driver/governor.

Yes right.

> 
> When there is no cpuidle driver, there is no instance which could bring
> the CPU into a deeper C state. But at the moment the code does
> unconditionally try to stop the tick. So the aim of the patch is to
> remove this unconditional stop of the tick.

I agree that the absence of cpuidle driver prevents from reaching deep
idle states. FWIU, there is however still benefits in stopping the tick
on such platform.
-
I agree that bringing up/down the ticks costs something and that removing
tick_nohz_idle_stop_tick() can improve performance, but I assumed stopping
the ticks had some interest regarding energy consumption.
Keeping the tick forever on an idle CPU should not be useful.
-
About nohz.idle_cpus_mask, I was referring to the following path:
do_idle()
\-cpuidle_idle_call()
   \-tick_nohz_idle_stop_tick()
     \-nohz_balance_enter_idle()
       \-cpumask_set_cpu(cpu, nohz.idle_cpus_mask);
       \-atomic_inc(&nohz.nr_cpus);

Removing tick_nohz_idle_stop_tick() also means not using nohz.idle_cpus_mask
and the logic around it to find an idle CPU to balance tasks.

Hope the re-phrasing makes the 2 points a bit clearer,
Regards,
Pierre


> 
> And NOHZ is independant on the cpuidle infrastructure. But when there is
> no cpuidle driver, it doesn't makes sense to use then also NOHZ.
> 
> Thanks,
> 
> 	Anna-Maria
> 
> 
>