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

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Date:	Thu, 7 May 2009 12:50:07 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	Balaji Rao <balajirrao@...il.com>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH v3] cpuacct: VIRT_CPU_ACCOUNTING don't prevent percpu
	cputime count


* KOSAKI Motohiro <kosaki.motohiro@...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@...ux.vnet.ibm.com>
> Cc: Balaji Rao <balajirrao@...il.com>
> Cc: Dhaval Giani <dhaval@...ux.vnet.ibm.com>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Cc: Balbir Singh <balbir@...ux.vnet.ibm.com>
> Cc: Ingo Molnar <mingo@...e.hu>
> Cc: Martin Schwidefsky <schwidefsky@...ibm.com>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...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
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