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References: <20210627072833.2199463-1-ani@anisinha.ca>
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From:   Ani Sinha <ani@anisinha.ca>
Date:   Sat, 3 Jul 2021 22:59:35 +0530
Message-ID: <CAARzgww4ZO3MtVxEM+yRNJ1Um-5DuVfUxh2+Hnjt5MKf-Rv8og@mail.gmail.com>
Subject: Re: [PATCH v1] Add info log when user enables NOHZ in commandline but
 NOHZ is not possible
To:     linux-kernel@vger.kernel.org
Cc:     anirban.sinha@nokia.com, Thomas Gleixner <tglx@linutronix.de>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Ingo Molnar <mingo@kernel.org>
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ping ...

On Sun, Jun 27, 2021 at 12:58 PM Ani Sinha <ani@anisinha.ca> wrote:
>
> Commit 4f49b90abb4aca ("sched-clock: Migrate to use new tick
> dependency mask model") had also removed the kernel warning
> message informing the user that it was not possible to turn
> on NO_HZ_FULL. This warning was added in commit e12d0271774fe
> ("nohz: Warn if the machine can not perform nohz_full") in order
> to warn users when they enabled NOHZ using the kernel commandline
> but kernel could not enable NOHZ. Removal of this log defeats this
> purpose because we still do not want users thinking that they are
> getting the benefit of nohz when their machine can not support it.
> Hence this patch brings back the information log.
>
> If at some point the conditions that prevented NOHZ from being
> enabled goes away and the previous info log was printed, we print
> another log informing the user that NOHZ was again possible.
>
> Signed-off-by: Ani Sinha <ani@anisinha.ca>
> ---
>  kernel/time/tick-sched.c | 21 ++++++++++++++++++---
>  1 file changed, 18 insertions(+), 3 deletions(-)
>
> See also thread: https://lkml.org/lkml/2021/6/20/32
>
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 828b091501ca..fbef95b57b1d 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -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;
> +               }
> +               if (ts->tick_stopped)
> +                       tick_nohz_restart_sched_tick(ts, ktime_get());
> +       }
>  #endif
>  }
>
> --
> 2.25.1
>