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SubjectRe: [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.

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