linux-kernel - Re: [PATCH] Add missing kernel log when enabling NO_HZ

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Date:   Sat, 19 Jun 2021 10:02:30 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ani Sinha <ani@...sinha.ca>, linux-kernel@...r.kernel.org
Cc:     anirban.sinha@...ia.com, Ani Sinha <ani@...sinha.ca>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] Add missing kernel log when enabling NO_HZ_FULL is not possible

Ani,

On Fri, Jun 11 2021 at 16:09, Ani Sinha wrote:
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 828b091501ca..a82480c036e2 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -937,10 +937,18 @@ static void tick_nohz_full_update_tick(struct tick_sched *ts)
>  	if (!ts->tick_stopped && ts->nohz_mode == NOHZ_MODE_INACTIVE)
>  		return;
>  
> -	if (can_stop_full_tick(cpu, ts))
> +	if (can_stop_full_tick(cpu, ts)) {
>  		tick_nohz_stop_sched_tick(ts, cpu);
> -	else if (ts->tick_stopped)
> -		tick_nohz_restart_sched_tick(ts, ktime_get());
> +	} else {
> +		/*
> +		 * Don't allow the user to think they can get
> +		 * full NO_HZ with this machine.
> +		 */
> +		WARN_ONCE(tick_nohz_full_running,
> +			  "NO_HZ_FULL will not work with current sched clock");

How is that warning useful and even remotely correct?

can_stop_full_tick() has 4 x 5 == 20 ways to return false and the
smaller portion of them is related to sched clock.

Thanks,

        tglx