linux-kernel - Re: [patch 1/2] sched, timers: move calc

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Message-Id: <1242331227.6642.1637.camel@laptop>
Date:	Thu, 14 May 2009 22:00:27 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dimitri Sivanich <sivanich@....com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [patch 1/2] sched, timers: move calc_load() to scheduler

On Thu, 2009-05-14 at 11:21 +0000, Thomas Gleixner wrote:
> plain text document attachment (move-calc-load-to-scheduler-v1.patch)

> +/*
> + * calc_load - update the avenrun load estimates 10 ticks after the
> + * CPUs have updated calc_load_tasks.
> + */
> +void calc_global_load(void)
> +{
> +	unsigned long upd = calc_load_update + 10;
> +	long active;
> +
> +	if (time_before(jiffies, upd))
> +		return;
>  
> +	active = atomic_long_read(&calc_load_tasks);
> +	active = active > 0 ? active * FIXED_1 : 0;
>  
> +	avenrun[0] = calc_load(avenrun[0], EXP_1, active);
> +	avenrun[1] = calc_load(avenrun[1], EXP_5, active);
> +	avenrun[2] = calc_load(avenrun[2], EXP_15, active);
> +
> +	calc_load_update += LOAD_FREQ;
> +}

> @@ -1211,7 +1160,8 @@ static inline void update_times(unsigned
>  void do_timer(unsigned long ticks)
>  {
>  	jiffies_64 += ticks;
> -	update_times(ticks);
> +	update_wall_time();
> +	calc_global_load();
>  }

I can see multiple cpus fall into calc_global_load() concurrently, which
would 'age' the load faster than expected.

Should we plug that hole?

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