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

Martin Schwidefsky
2010-01-18T09:42:23Z

Hi Anton,

On Mon, 18 Jan 2010 15:41:42 +1100
Anton Blanchard <anton@samba.org> wrote:

> Note: ccing ia64 and s390 who have not yet added code to statically
> initialise cputime_one_jiffy at boot.
> See a42548a18866e87092db93b771e6c5b060d78401 (cputime: Optimize
> jiffies_to_cputime(1) for details). Adding this would help optimise not only
> this patch but many other areas of the scheduler when
> CONFIG_VIRT_CPU_ACCOUNTING is enabled.

For s390 the jiffies_to_cputime is a compile time constant. No need to
initialize it at runtime, no?

> Index: linux.trees.git/kernel/sched.c
> ===================================================================
> --- linux.trees.git.orig/kernel/sched.c 2010-01-18 14:27:12.000000000 +1100
> +++ linux.trees.git/kernel/sched.c 2010-01-18 15:21:59.000000000 +1100
> @@ -10894,6 +10894,7 @@ static void cpuacct_update_stats(struct
> enum cpuacct_stat_index idx, cputime_t val)
> {
> struct cpuacct *ca;
> + int batch;
>
> if (unlikely(!cpuacct_subsys.active))
> return;
> @@ -10901,8 +10902,9 @@ static void cpuacct_update_stats(struct
> 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);
> + __percpu_counter_add(&ca->cpustat[idx], val, batch);
> ca = ca->parent;
> } while (ca);
> rcu_read_unlock();

The patch itself trades some accuracy (larger cpu accounting value that
are stored per-cpu) against runtime overhead (spinlock to transfer the
value to the global variable in __percpu_counter_add). Did you
calculate how big the loss in accuracy is?

--
blue skies,
Martin.

"Reality continues to ruin my life." - Calvin.