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

Andrew Morton
2010-01-25T23:14:54Z

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?