linux-kernel - Re: [PATCH 02/32] cpuset: Set up interface for nohz flag

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Date:	Tue, 30 Oct 2012 13:16:14 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	linux-kernel@...r.kernel.org
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Clark Williams <clark.williams@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Li Zefan <lizf@...fujitsu.com>, Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Avi Kivity <avi@...hat.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Kevin Hilman <khilman@...com>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Sven-Thorsten Dietrich <thebigcorporation@...il.com>
Subject: Re: [PATCH 02/32] cpuset: Set up interface for nohz flag

On Mon, 2012-10-29 at 16:27 -0400, Steven Rostedt wrote:

>  #ifdef CONFIG_CPUSETS
>  
> @@ -235,4 +236,34 @@ static inline bool put_mems_allowed(unsigned int seq)
>  
>  #endif /* !CONFIG_CPUSETS */
>  
> +#ifdef CONFIG_CPUSETS_NO_HZ
> +
> +DECLARE_PER_CPU(atomic_t, cpu_adaptive_nohz_ref);
> +
> +static inline bool cpuset_cpu_adaptive_nohz(int cpu)
> +{
> +	atomic_t *ref = &per_cpu(cpu_adaptive_nohz_ref, cpu);
> +
> +	if (atomic_add_return(0, ref) > 0)

I'm assuming you do the atomic_add_return() for the implicit memory
barrier? Yuck!

Please comment this. I see that rcutree.c does the same thing without a
comment. Bad Paul, bad!


> +		return true;
> +
> +	return false;
> +}
> +
> +static inline bool cpuset_adaptive_nohz(void)
> +{
> +	/*
> +	 * We probably want to do atomic_read() when we read
> +	 * locally to avoid the overhead of an ordered add.
> +	 * For that we have to do the dec of the ref locally as
> +	 * well.

Does it matter if we miss the dec? What other synchronization is used?

	CPU 1					CPU 2
	------					-----
	var = atomic_add_return(0, ref)
						atomic_dec(ref);
	if (var > 0)

returns true.

For local cases, as this seems to be in a fast path, we should use
this_cpu_read() as well.

	
> +	 */
> +	return cpuset_cpu_adaptive_nohz(smp_processor_id());
> +}
> +#else
> +static inline bool cpuset_cpu_adaptive_nohz(int cpu) { return false; }
> +static inline bool cpuset_adaptive_nohz(void) { return false; }
> +
> +#endif /* CONFIG_CPUSETS_NO_HZ */
> +
>  #endif /* _LINUX_CPUSET_H */
> diff --git a/init/Kconfig b/init/Kconfig
> index 6fdd6e3..ffdeeab 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -749,6 +749,14 @@ config PROC_PID_CPUSET
>  	depends on CPUSETS
>  	default y
>  
> +config CPUSETS_NO_HZ
> +       bool "Tickless cpusets"
> +       depends on CPUSETS && HAVE_CPUSETS_NO_HZ
> +       help
> +         This options let you apply a nohz property to a cpuset such
> +	 that the periodic timer tick tries to be avoided when possible on
> +	 the concerned CPUs.
> +
>  config CGROUP_CPUACCT
>  	bool "Simple CPU accounting cgroup subsystem"
>  	help
> diff --git a/kernel/cpuset.c b/kernel/cpuset.c
> index f33c715..6319d8e 100644
> --- a/kernel/cpuset.c
> +++ b/kernel/cpuset.c
> @@ -145,6 +145,7 @@ typedef enum {
>  	CS_SCHED_LOAD_BALANCE,
>  	CS_SPREAD_PAGE,
>  	CS_SPREAD_SLAB,
> +	CS_ADAPTIVE_NOHZ,
>  } cpuset_flagbits_t;
>  
>  /* the type of hotplug event */
> @@ -189,6 +190,11 @@ static inline int is_spread_slab(const struct cpuset *cs)
>  	return test_bit(CS_SPREAD_SLAB, &cs->flags);
>  }
>  
> +static inline int is_adaptive_nohz(const struct cpuset *cs)
> +{
> +	return test_bit(CS_ADAPTIVE_NOHZ, &cs->flags);
> +}

We can move this into the #ifdef CONFIG_CPUSETS_NO_HZ as well, and have
the #else version just return zero. Why use test_bit() when we already
know the answer?

> +
>  static struct cpuset top_cpuset = {
>  	.flags = ((1 << CS_CPU_EXCLUSIVE) | (1 << CS_MEM_EXCLUSIVE)),
>  };
> @@ -1190,6 +1196,32 @@ static void cpuset_change_flag(struct task_struct *tsk,
>  	cpuset_update_task_spread_flag(cgroup_cs(scan->cg), tsk);
>  }
>  
> +#ifdef CONFIG_CPUSETS_NO_HZ
> +
> +DEFINE_PER_CPU(atomic_t, cpu_adaptive_nohz_ref);
> +
> +static void update_nohz_cpus(struct cpuset *old_cs, struct cpuset *cs)
> +{
> +	int cpu;
> +	int val;
> +
> +	if (is_adaptive_nohz(old_cs) == is_adaptive_nohz(cs))
> +		return;
> +
> +	for_each_cpu(cpu, cs->cpus_allowed) {
> +		atomic_t *ref = &per_cpu(cpu_adaptive_nohz_ref, cpu);
> +		if (is_adaptive_nohz(cs))
> +			atomic_inc(ref);
> +		else
> +			atomic_dec(ref);
> +	}
> +}
> +#else
> +static inline void update_nohz_cpus(struct cpuset *old_cs, struct cpuset *cs)
> +{
> +}
> +#endif
> +

-- Steve


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