linux-kernel - Re: [PATCH] nohz: Do not warn about unstable tsc unless user uses nohz

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Date:	Tue, 16 Jul 2013 18:12:29 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Dave Jones <davej@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] nohz: Do not warn about unstable tsc unless user uses
 nohz_full

On Tue, Jul 16, 2013 at 10:22:12AM -0400, Steven Rostedt wrote:
> If the user enables CONFIG_NO_HZ_FULL and runs the kernel on a machine
> with an unstable TSC, it will produce a WARN_ON dump as well as taint
> the kernel. This is a bit extreme for a kernel that just enables a
> feature but doesn't use it.
> 
> The warning should only happen if the user tries to use the feature by
> either adding nohz_full to the kernel command line, or by enabling
> CONFIG_NO_HZ_FULL_ALL that makes nohz used on all CPUs at boot up. Note,
> this second feature should not (yet) be used by distros or anyone that
> doesn't care if NO_HZ is used or not.
> 
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> 
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 6960172..6f47049 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -182,7 +182,8 @@ static bool can_stop_full_tick(void)
>  		 * Don't allow the user to think they can get
>  		 * full NO_HZ with this machine.
>  		 */
> -		WARN_ONCE(1, "NO_HZ FULL will not work with unstable sched clock");
> +		WARN_ONCE(have_nohz_full_mask,
> +			  "NO_HZ FULL will not work with unstable sched clock");

Seems good, indeed the warning and taint can be justified if the user filled the
nohz_full mask.

Ok I'll apply this and let the last word to Ingo in a pull request.

Thanks.

>  		return false;
>  	}
>  #endif
> 
> 
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