linux-kernel - Re: [PATCH] sched: Notify hrtimer users of switches to NOHZ mode

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Date:	Wed, 19 Jan 2011 12:43:40 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	tglx <tglx@...utronix.de>
Subject: Re: [PATCH] sched: Notify hrtimer users of switches to NOHZ mode

On Tue, 2011-01-18 at 22:46 -0800, Stephen Boyd wrote:
> When NOHZ=y and high res timers are disabled (via cmdline or
> Kconfig) tick_nohz_switch_to_nohz() will notify the user about
> switching into NOHZ mode. Nothing is printed for the case where
> HIGH_RES_TIMERS=y. Fix this for the HIGH_RES_TIMERS=y case by
> duplicating the printk from the low res NOHZ path in the high res
> NOHZ path.
> 
> Signed-off-by: Stephen Boyd <sboyd@...eaurora.org>
> ---
> 
> This confused me since I was thinking 'dmesg | grep -i NOHZ' would
> tell me if NOHZ was enabled, but if I have hrtimers there is
> nothing. If we don't do this we should probably remove the original
> printk.

I'm not quite sure we want _more_ dmesg chatter, how about you look at
the output of:

grep -e "^cpu" -e "nohz" /proc/timer_list

instead?

>  kernel/time/tick-sched.c |    5 ++++-
>  1 files changed, 4 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 3e216e0..6c012c5 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -795,8 +795,11 @@ void tick_setup_sched_timer(void)
>  	}
>  
>  #ifdef CONFIG_NO_HZ
> -	if (tick_nohz_enabled)
> +	if (tick_nohz_enabled) {
>  		ts->nohz_mode = NOHZ_MODE_HIGHRES;
> +		pr_info("Switched to NOHz mode on CPU #%d\n",
> +				smp_processor_id());
> +	}
>  #endif
>  }
>  #endif /* HIGH_RES_TIMERS */


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