linux-kernel - Re: [PATCH v1] Add info log when user enables NOHZ in commandline but NOHZ is not possible

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Date:   Wed, 07 Jul 2021 12:16:15 +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 v1] Add info log when user enables NOHZ in commandline but NOHZ is not possible

Ani,

On Sun, Jun 27 2021 at 12:58, Ani Sinha wrote:

same comment vs. Subject, 'this patch' and 'we'

> @@ -930,6 +930,7 @@ static void tick_nohz_full_update_tick(struct tick_sched *ts)
>  {
>  #ifdef CONFIG_NO_HZ_FULL
>  	int cpu = smp_processor_id();
> +	static bool no_tick_warned;
>  
>  	if (!tick_nohz_full_cpu(cpu))
>  		return;
> @@ -937,10 +938,24 @@ 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());
> +		if (no_tick_warned) {
> +			pr_info("NO_HZ_FULL is now enabled in the system.\n");
> +			no_tick_warned = false;
> +		}
> +	} else {
> +		/*
> +		 * Don't allow the user to think they can get
> +		 * full NO_HZ with this machine.
> +		 */
> +		if (!no_tick_warned && tick_nohz_full_running) {
> +			pr_info("NO_HZ_FULL has been disabled in the system.");
> +			no_tick_warned = true;
> +		}

So this is going to emit this message everytime the NOHZ state of a CPU
changes, which is an easy to trigger dmesg flooding from unpriviledged
user space.

Also the message is misleading because this is a per CPU condition and
not a system wide condition.

Thanks,

        tglx