linux-kernel - Re: [PATCH 07/33] nohz: Basic full dynticks interface

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Message-ID: <20130211143529.GB2749@pd.tnic>
Date:	Mon, 11 Feb 2013 15:35:29 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung.kim@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 07/33] nohz: Basic full dynticks interface

On Tue, Jan 08, 2013 at 03:08:07AM +0100, Frederic Weisbecker wrote:

[ … ]

> diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
> index 8601f0d..dc6381d 100644
> --- a/kernel/time/Kconfig
> +++ b/kernel/time/Kconfig
> @@ -70,6 +70,15 @@ config NO_HZ
>  	  only trigger on an as-needed basis both when the system is
>  	  busy and when the system is idle.
>  
> +config NO_HZ_FULL
> +       bool "Full tickless system"

I think you want to say here "Almost-completely tickless system".
"Almost" because of that one CPU outside of the range :-)

> +       depends on NO_HZ && RCU_USER_QS && VIRT_CPU_ACCOUNTING_GEN && RCU_NOCB_CPU && SMP
> +       select CONTEXT_TRACKING_FORCE
> +       help
> +         Try to be tickless everywhere, not just in idle. (You need
> +	 to fill up the full_nohz_mask boot parameter).
> +
> +
>  config HIGH_RES_TIMERS
>  	bool "High Resolution Timer Support"
>  	depends on !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 314b9ee..494a2aa 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -142,6 +142,29 @@ static void tick_sched_handle(struct tick_sched *ts, struct pt_regs *regs)
>  	profile_tick(CPU_PROFILING);
>  }
>  
> +#ifdef CONFIG_NO_HZ_FULL
> +static cpumask_var_t full_nohz_mask;
> +bool have_full_nohz_mask;
> +
> +int tick_nohz_full_cpu(int cpu)
> +{
> +	if (!have_full_nohz_mask)
> +		return 0;
> +
> +	return cpumask_test_cpu(cpu, full_nohz_mask);
> +}
> +
> +/* Parse the boot-time nohz CPU list from the kernel parameters. */
> +static int __init tick_nohz_full_setup(char *str)
> +{
> +	alloc_bootmem_cpumask_var(&full_nohz_mask);
> +	have_full_nohz_mask = true;
> +	cpulist_parse(str, full_nohz_mask);

Don't you want to check retval of cpulist_parse first here before
assigning have_full_nohz_mask and allocating cpumask var?

We don't trust userspace, you know.

> +	return 1;
> +}
> +__setup("full_nohz=", tick_nohz_full_setup);

I'd guess this kernel parameter needs to go into
Documentation/kernel-parameters.txt along with a referral to
Documentation/cputopology.txt which explains how to specify cpulists for
n00bs like me :-)

Thanks.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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