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Date: Wed, 6 Apr 2016 08:54:36 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: tglx@linutronix.de, efault@gmx.de, htejun@gmail.com,
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        torvalds@linux-foundation.org, zhaolei@cn.fujitsu.com,
        peterz@infradead.org, yangds.fnst@cn.fujitsu.com, hpa@zytor.com
Subject: Re: [tip:sched/core] sched/cpuacct: Split usage accounting into
 user_usage and sys_usage
Message-ID: <20160406065436.GB3078@gmail.com>
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 <tip-d740037fac7052e49450f6fa1454f1144a103b55@git.kernel.org>
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* Srikar Dronamraju <srikar@linux.vnet.ibm.com> wrote:

> * tip-bot for Dongsheng Yang <tipbot@zytor.com> [2016-03-31 02:27:39]:
> 
> > Commit-ID:  d740037fac7052e49450f6fa1454f1144a103b55
> > Gitweb:     http://git.kernel.org/tip/d740037fac7052e49450f6fa1454f1144a103b55
> > Author:     Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
> > AuthorDate: Tue, 22 Mar 2016 16:37:08 +0800
> > Committer:  Ingo Molnar <mingo@kernel.org>
> > CommitDate: Thu, 31 Mar 2016 10:48:54 +0200
> > 
> > sched/cpuacct: Split usage accounting into user_usage and sys_usage
> > 
> > Sometimes, cpuacct.usage is not detailed enough to see how much CPU
> > usage a group had. We want to know how much time it used in user mode
> > and how much in kernel mode.
> > 
> 
> Unfortunately this commit causes boot to fail on my power 7 box.
> 
> > @@ -238,10 +316,18 @@ static struct cftype files[] = {
> >  void cpuacct_charge(struct task_struct *tsk, u64 cputime)
> >  {
> >  	struct cpuacct *ca;
> > +	int index;
> > +
> > +	if (user_mode(task_pt_regs(tsk)))
> > +		index = CPUACCT_USAGE_USER;
> > +	else
> > +		index = CPUACCT_USAGE_SYSTEM;
> >  
> >  	rcu_read_lock();
> > +
> >  	for (ca = task_ca(tsk); ca; ca = parent_ca(ca))
> > -		*this_cpu_ptr(ca->cpuusage) += cputime;
> > +		this_cpu_ptr(ca->cpuusage)->usages[index] += cputime;
> 
> The above line seems to be the cause of oops. Attached the complete console log below.

Weird - not much changed wrt. the cpuusage logic, we only increased its size.

If you change the above loop to something like:

	for (ca = task_ca(tsk); ca; ca = parent_ca(ca)) {
		if (WARN_ON_ONCE(!ca->cpuusage))
			continue;
		this_cpu_ptr(ca->cpuusage)->usages[index] += cputime;
	}

then do you get the warning and the bootup succeeds?

Thanks,

	Ingo