Re: [PATCH] sched: cpuacct: Use bigger percpu counter batch valuesfor stats counters

[Andrew Morton]

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