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, 7 Jul 2021 12:43:53 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Ani Sinha <ani@...sinha.ca>, linux-kernel@...r.kernel.org,
        anirban.sinha@...ia.com, 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

On Wed, Jul 07, 2021 at 12:16:15PM +0200, Thomas Gleixner wrote:
> 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.

IIUC, the clock can be marked unstable anytime before or after nohz_full
is initialized.

So we can do two things:

* If tick_nohz_init() is called after the clock has been marked unstable,
  just don't initialize nohz_full.

* If the clock is marked unstable after tick_nohz_init(), issue a pr_warn()
  from __clear_sched_clock_stable().

Thanks.