Date: Mon, 25 Jan 2010 15:14:04 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Anton Blanchard <anton@samba.org>
Cc: Bharata B Rao <bharata@linux.vnet.ibm.com>,
       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       Ingo Molnar <mingo@elte.hu>, Balbir Singh <balbir@linux.vnet.ibm.com>,
       mingo@redhat.com, hpa@zytor.com, linux-kernel@vger.kernel.org,
       a.p.zijlstra@chello.nl, schwidefsky@de.ibm.com, balajirrao@gmail.com,
       dhaval@linux.vnet.ibm.com, tglx@linutronix.de,
       kamezawa.hiroyu@jp.fujitsu.com, Tony Luck <tony.luck@intel.com>,
       Fenghua Yu <fenghua.yu@intel.com>,
       Heiko Carstens <heiko.carstens@de.ibm.com>, linux390@de.ibm.com
Subject: Re: [PATCH] sched: cpuacct: Use bigger percpu counter batch values
 for stats counters
Message-Id: <20100125151404.7b7e6970.akpm@linux-foundation.org>
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On Mon, 18 Jan 2010 15:41:42 +1100
Anton Blanchard <anton@samba.org> wrote:

> When CONFIG_VIRT_CPU_ACCOUNTING and CONFIG_CGROUP_CPUACCT are enabled we can
> call cpuacct_update_stats with values much larger than percpu_counter_batch.
> This means the call to percpu_counter_add will always add to the global count
> which is protected by a spinlock and we end up with a global spinlock in
> the scheduler.

When one looks at the end result:

: static void cpuacct_update_stats(struct task_struct *tsk,
: 		enum cpuacct_stat_index idx, cputime_t val)
: {
: 	struct cpuacct *ca;
: 	int batch;
: 
: 	if (unlikely(!cpuacct_subsys.active))
: 		return;
: 
: 	rcu_read_lock();
: 	ca = task_ca(tsk);
: 
: 	batch = min_t(long, percpu_counter_batch * cputime_one_jiffy, INT_MAX);
: 	do {
: 		__percpu_counter_add(&ca->cpustat[idx], val, batch);
: 		ca = ca->parent;
: 	} while (ca);
: 	rcu_read_unlock();
: }

the code (which used to be quite obvious) becomes pretty unobvious.  In
fact it looks quite wrong.

Shouldn't there be a comment there explaining wtf is going on?
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