linux-kernel - Re: [PATCH 06/33] cputime: Safely read cputime of full dynticks CPUs

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Message-ID: <20130109183454.GB8293@somewhere.redhat.com>
Date:	Wed, 9 Jan 2013 19:35:06 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
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 06/33] cputime: Safely read cputime of full dynticks CPUs

On Wed, Jan 09, 2013 at 09:54:11AM -0500, Steven Rostedt wrote:
> On Tue, 2013-01-08 at 03:08 +0100, Frederic Weisbecker wrote:
> 
> > diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> > index 07912dd..bf4f72d 100644
> > --- a/kernel/sched/cputime.c
> > +++ b/kernel/sched/cputime.c
> > @@ -484,7 +484,7 @@ void vtime_task_switch(struct task_struct *prev)
> >   * vtime_account().
> >   */
> >  #ifndef __ARCH_HAS_VTIME_ACCOUNT
> > -void vtime_account(struct task_struct *tsk)
> > +void vtime_account_irq_enter(struct task_struct *tsk)
> >  {
> >  	if (!in_interrupt()) {
> >  		/*
> > @@ -505,7 +505,7 @@ void vtime_account(struct task_struct *tsk)
> >  	}
> >  	vtime_account_system(tsk);
> >  }
> > -EXPORT_SYMBOL_GPL(vtime_account);
> > +EXPORT_SYMBOL_GPL(vtime_account_irq_enter);
> >  #endif /* __ARCH_HAS_VTIME_ACCOUNT */
> >  #endif /* CONFIG_VIRT_CPU_ACCOUNTING */
> >  
> > @@ -616,41 +616,67 @@ void thread_group_cputime_adjusted(struct task_struct *p, cputime_t *ut, cputime
> >  #endif /* !CONFIG_VIRT_CPU_ACCOUNTING_NATIVE */
> >  
> >  #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
> > -static DEFINE_PER_CPU(long, last_jiffies) = INITIAL_JIFFIES;
> > -
> > -static cputime_t get_vtime_delta(void)
> > +static cputime_t get_vtime_delta(struct task_struct *tsk)
> >  {
> >  	long delta;
> >  
> > -	delta = jiffies - __this_cpu_read(last_jiffies);
> > -	__this_cpu_add(last_jiffies, delta);
> > +	delta = jiffies - tsk->prev_jiffies;
> > +	tsk->prev_jiffies += delta;
> >  
> >  	return jiffies_to_cputime(delta);
> >  }
> >  
> > -void vtime_account_system(struct task_struct *tsk)
> > +static void __vtime_account_system(struct task_struct *tsk)
> >  {
> > -	cputime_t delta_cpu = get_vtime_delta();
> > +	cputime_t delta_cpu = get_vtime_delta(tsk);
> >  
> >  	account_system_time(tsk, irq_count(), delta_cpu, cputime_to_scaled(delta_cpu));
> >  }
> >  
> > +void vtime_account_system(struct task_struct *tsk)
> > +{
> > +	write_seqlock(&tsk->vtime_seqlock);
> > +	__vtime_account_system(tsk);
> 
> __vtime_account_system() calls account_system_time()
> account_system_time() calls __account_system_time()
> __account_system_time() calls acct_update_integrals()
> (when CONFIG_TASK_XACCT is set)
> acct_update_integrals() calls task_cputime()
> task_cputime() grabs t->vtime_seqlock for read
> 
>  DEADLOCK
> 
> ironically the subject says *Safely* read cputime ;-)

Well at least it crashes safely. Safely as in "deterministic".
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