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

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Message-ID: <20100118104213.64231c1e@mschwide.boeblingen.de.ibm.com>
Date:	Mon, 18 Jan 2010 10:42:13 +0100
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Anton Blanchard <anton@...ba.org>
Cc:	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>, mingo@...hat.com,
	hpa@...or.com, linux-kernel@...r.kernel.org,
	a.p.zijlstra@...llo.nl, balajirrao@...il.com,
	dhaval@...ux.vnet.ibm.com, tglx@...utronix.de,
	kamezawa.hiroyu@...fujitsu.com, akpm@...ux-foundation.org,
	Tony Luck <tony.luck@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>, linux390@...ibm.com
Subject: Re: [PATCH] sched: cpuacct: Use bigger percpu counter batch values
 for stats counters

Hi Anton,

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

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