linux-kernel - Re: [PATCH v3] time/tick-sched: idle load balancing when nohz

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Date: Thu, 9 May 2024 08:28:21 +0200
From: Markus Elfring <Markus.Elfring@....de>
To: Levi Yun <ppbuk5246@...il.com>, kernel-janitors@...r.kernel.org,
 Anna-Maria Behnsen <anna-maria@...utronix.de>,
 Frederic Weisbecker <frederic@...nel.org>, Ingo Molnar <mingo@...nel.org>,
 Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] time/tick-sched: idle load balancing when nohz_full
 cpu becomes idle.

> When nohz_full CPU stops tick in tick_nohz_irq_exit(),
> It wouldn't be chosen to perform idle load balancing because it doesn't
> call nohz_balance_enter_idle() in tick_nohz_idle_stop_tick() when it
> becomes idle.
>
> tick_nohz_idle_stop_tick() is only called in idle state and
> nohz_balance_enter_idle() tracks the CPU which is part of nohz.idle_cpus_mask
> with rq->nohz_tick_stopped.
>
> Change tick_nohz_idle_stop_tick() to call nohz_balance_enter_idle()
> without checking !was_stopped so that nohz_full cpu can be chosen to
> perform idle load balancing when it enters idle state.

Would you eventually like to add the tag “Fixes” once more?

…
> +++ b/kernel/time/tick-sched.c
> @@ -1228,8 +1228,10 @@ void tick_nohz_idle_stop_tick(void)
>  		ts->idle_sleeps++;
>  		ts->idle_expires = expires;
>
> -		if (!was_stopped && tick_sched_flag_test(ts, TS_FLAG_STOPPED)) {
> -			ts->idle_jiffies = ts->last_jiffies;
> +		if (tick_sched_flag_test(ts, TS_FLAG_STOPPED)) {
> +			if (!was_stopped)
> +				ts->idle_jiffies = ts->last_jiffies;
> +
>  			nohz_balance_enter_idle(cpu);
>  		}
…

I interpret these diff data in the way that you propose to reorder
two condition checks.

But I wonder still how “good” the presented change description fits to
the suggested source code adjustment.

Regards,
Markus