linux-kernel - Re: [PATCH 4/5] sched: Refactor iowait accounting

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Date:	Sat, 19 Oct 2013 17:35:35 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Fernando Luis Vazquez Cao <fernando_b1@....ntt.co.jp>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: [PATCH 4/5] sched: Refactor iowait accounting

On Sat, Oct 19, 2013 at 05:17:20PM +0200, Frederic Weisbecker wrote:
> +u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time)
> +{
> +	ktime_t iowait, delta = { .tv64 = 0 };
> +	struct rq *rq = cpu_rq(cpu);
> +	ktime_t now = ktime_get();
> +	unsigned int seq;
> +
> +	do {
> +		seq = read_seqbegin(&rq->iowait_lock);
> +		if (rq->nr_iowait)
> +			delta = ktime_sub(now, rq->iowait_start);
> +		iowait = ktime_add(rq->iowait_time, delta);
> +	} while (read_seqretry(&rq->iowait_lock, seq));
> +
> +	if (last_update_time)
> +		*last_update_time = ktime_to_us(now);
> +
> +	return ktime_to_us(iowait);
> +}
> +EXPORT_SYMBOL_GPL(get_cpu_iowait_time_us);
> +
> +static void cpu_iowait_start(struct rq *rq)
> +{
> +	write_seqlock(&rq->iowait_lock);
> +	if (!rq->nr_iowait++)
> +		rq->iowait_start = ktime_get();
> +	write_sequnlock(&rq->iowait_lock);
> +}
> +
> +static void cpu_iowait_end(struct rq *rq)
> +{
> +	ktime_t delta;
> +	write_seqlock(&rq->iowait_lock);
> +	if (!--rq->nr_iowait) {
> +		delta = ktime_sub(ktime_get(), rq->iowait_start);
> +		rq->iowait_time = ktime_add(rq->iowait_time, delta);
> +	}
> +	write_sequnlock(&rq->iowait_lock);
> +}

Yeah, so using ktime_get() for this is completely insane ;-)

I just had a look at delayacct; wth wrote that crap; that too uses
gtod.

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