linux-kernel - Re: [PATCH] sched: cpuacct: Use bigger percpu counter batch values for stats counters

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Date:	Mon, 25 Jan 2010 15:14:04 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Anton Blanchard <anton@...ba.org>
Cc:	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>, mingo@...hat.com,
	hpa@...or.com, linux-kernel@...r.kernel.org,
	a.p.zijlstra@...llo.nl, schwidefsky@...ibm.com,
	balajirrao@...il.com, dhaval@...ux.vnet.ibm.com,
	tglx@...utronix.de, kamezawa.hiroyu@...fujitsu.com,
	Tony Luck <tony.luck@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>, linux390@...ibm.com
Subject: Re: [PATCH] sched: cpuacct: Use bigger percpu counter batch values
 for stats counters

On Mon, 18 Jan 2010 15:41:42 +1100
Anton Blanchard <anton@...ba.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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