linux-kernel - Re: [RFC PATCH -tip] cpuacct: per-cgroup utime/stime statistics

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Date:	Mon, 16 Mar 2009 10:07:54 +0530
From:	Bharata B Rao <bharata@...ux.vnet.ibm.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org, Balaji Rao <balajirrao@...il.com>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Paul Menage <menage@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-arch@...r.kernel.org
Subject: Re: [RFC PATCH -tip] cpuacct: per-cgroup utime/stime statistics -
	v2

On Mon, Mar 16, 2009 at 10:35:17AM +0900, KAMEZAWA Hiroyuki wrote:
> On Thu, 12 Mar 2009 16:39:24 +0530
> Bharata B Rao <bharata@...ux.vnet.ibm.com> wrote:
> 
> >  #ifdef CONFIG_CGROUP_CPUACCT
> >  static void cpuacct_charge(struct task_struct *tsk, u64 cputime);
> > +static void cpuacct_update_stats(struct task_struct *tsk,
> > +		enum cpuacct_stat_index idx, cputime_t val);
> >  #else
> >  static inline void cpuacct_charge(struct task_struct *tsk, u64 cputime) {}
> > +static void cpuacct_update_stats(struct task_struct *tsk,
> > +		enum cpuacct_stat_index idx, cputime_t val) {}
> >  #endif
> "static inline"  
> if no "inline", the complier will show warning as
> "this function is defined but not used.."

Ok.

> >  
> > +/*
> > + * Account the system/user time to the task's accounting group.
> > + */
> > +static void cpuacct_update_stats(struct task_struct *tsk,
> > +		enum cpuacct_stat_index idx, cputime_t val)
> > +{
> > +	struct cpuacct *ca;
> > +
> > +	if (unlikely(!cpuacct_subsys.active))
> > +		return;
> > +
> > +	ca = task_ca(tsk);
> > +
> > +	do {
> > +		percpu_counter_add(&ca->cpustat[idx], val);
> > +		ca = ca->parent;
> > +	} while (ca);
> > +}
> > +
> 
> IIUC, to make sure accessing "ca" to be safe, we need some condition.
> (task_lock() or some other.....

task_lock() protects tsk->cgroups->subsys[]. So can we hold task_lock()
to protect this walk ? But we do this cpuacct hierarchy walk for the
current task here. So can a current task's ca or ca's parents disappear
from under us ?

Regards,
Bharata.
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