linux-kernel - Re: [PATCH 03/33] cputime: Generic on-demand virtual cputime accounting

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Date:	Tue, 08 Jan 2013 15:45:12 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
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>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 03/33] cputime: Generic on-demand virtual cputime
 accounting

On Tue, 2013-01-08 at 03:08 +0100, Frederic Weisbecker wrote:

> +#ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
> +static DEFINE_PER_CPU(long, last_jiffies) = INITIAL_JIFFIES;
> +
> +static cputime_t get_vtime_delta(void)
> +{
> +	long delta;
> +
> +	delta = jiffies - __this_cpu_read(last_jiffies);
> +	__this_cpu_add(last_jiffies, delta);
> +
> +	return jiffies_to_cputime(delta);
> +}
> +
> +void vtime_account_system(struct task_struct *tsk)
> +{
> +	cputime_t delta_cpu = get_vtime_delta();
> +
> +	account_system_time(tsk, irq_count(), delta_cpu, cputime_to_scaled(delta_cpu));
> +}
> +
> +void vtime_account_user(struct task_struct *tsk)
> +{
> +	cputime_t delta_cpu = get_vtime_delta();
> +
> +	/*
> +	 * This is an unfortunate hack: if we flush user time only on
> +	 * irq entry, we miss the jiffies update and the time is spuriously
> +	 * accounted to system time.
> +	 */
> +	if (context_tracking_in_user())
> +		account_user_time(tsk, delta_cpu, cputime_to_scaled(delta_cpu));

Hmm, you called get_vtime_delta() up above, which updates the
last_jiffies per_cpu variable, but if for some reason,
context_tracking_in_user() isn't true, we throw away the delta. What
happens to those lost jiffies? They go nowhere.

Shouldn't it be? :

	if (context_tracking_in_user()) {
		delta_cpu = get_vtime_delta();
		account_user_time(tsk, delta_cpu, cputime_to_scaled(delta_cpu));
	}

-- Steve

> +}
> +
> +void vtime_account_idle(struct task_struct *tsk)
> +{
> +	cputime_t delta_cpu = get_vtime_delta();
> +
> +	account_idle_time(delta_cpu);
> +}
> +
> +static int __cpuinit vtime_cpu_notify(struct notifier_block *self,
> +				      unsigned long action, void *hcpu)
> +{
> +	long cpu = (long)hcpu;
> +	long *last_jiffies_cpu = per_cpu_ptr(&last_jiffies, cpu);
> +
> +	switch (action) {
> +	case CPU_UP_PREPARE:
> +	case CPU_UP_PREPARE_FROZEN:
> +		/*
> +		 * CHECKME: ensure that's visible by the CPU
> +		 * once it wakes up
> +		 */
> +		*last_jiffies_cpu = jiffies;
> +	default:
> +		break;
> +	}
> +
> +	return NOTIFY_OK;
> +}
> +
> +static int __init init_vtime(void)
> +{
> +	cpu_notifier(vtime_cpu_notify, 0);
> +	return 0;
> +}
> +early_initcall(init_vtime);
> +#endif /* CONFIG_VIRT_CPU_ACCOUNTING_GEN */


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