Re: [PATCH v3] cpuacct: VIRT_CPU_ACCOUNTING don't prevent percpu cputime count

Ingo Molnar
2009-05-07T10:50:42Z

* KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:

>
> Changelog:
> since v2
> - revert using percpu_counter_sum()
>
> since v1
> - use percpu_counter_sum() instead percpu_counter_read()
>
>
> -------------------------------------
> Subject: [PATCH v3] cpuacct: VIRT_CPU_ACCOUNTING don't prevent percpu cputime count
>
> cpuacct_update_stats() is called at every tick updating. and it use percpu_counter
> for avoiding performance degression.
>
> For archs which define VIRT_CPU_ACCOUNTING, every tick would result
> in >1000 units of cputime updates and since this is much much greater
> than percpu_batch_counter, we end up taking spinlock on every tick.
>
> This patch change batch rule. now, any cpu can store "percpu_counter_bach * jiffies"
> cputime in per-cpu cache.
> it mean this patch don't have behavior change if VIRT_CPU_ACCOUNTING=n.
>
> Cc: Bharata B Rao <bharata@linux.vnet.ibm.com>
> Cc: Balaji Rao <balajirrao@gmail.com>
> Cc: Dhaval Giani <dhaval@linux.vnet.ibm.com>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> ---
> kernel/sched.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> Index: b/kernel/sched.c
> ===================================================================
> --- a/kernel/sched.c 2009-04-30 11:37:47.000000000 +0900
> +++ b/kernel/sched.c 2009-05-07 16:34:00.000000000 +0900
> @@ -10221,6 +10221,7 @@ struct cpuacct {
> };
>
> struct cgroup_subsys cpuacct_subsys;
> +static s32 cpuacct_batch;

should be __read_mostly i guess.

>
> /* return cpu accounting group corresponding to this container */
> static inline struct cpuacct *cgroup_ca(struct cgroup *cgrp)
> @@ -10250,6 +10251,9 @@ static struct cgroup_subsys_state *cpuac
> if (!ca->cpuusage)
> goto out_free_ca;
>
> + if (!cpuacct_batch)
> + cpuacct_batch = jiffies_to_cputime(percpu_counter_batch);

So this is a once per boot condition? Why not initialize it in
sched_init() or so? (instead of incuring this ugly check all the
time)

Ingo