linux-kernel - Re: [PATCH] Track accurate idle time with tick_sched.idle

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Message-ID: <p73bqcmvxc6.fsf@bingen.suse.de>
Date:	02 Sep 2007 00:23:53 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Venki Pallipadi <venkatesh.pallipadi@...el.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Track accurate idle time with tick_sched.idle_sleeptime

Venki Pallipadi <venkatesh.pallipadi@...el.com> writes:
>  
> +void tick_nohz_stop_idle(int cpu)
> +{
> +	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
> +
> +	if (ts->idle_active) {
> +		ktime_t now, delta;
> +		now = ktime_get();

That could be PM timer read costing thousands of cycles.

For going into idle it's probably not a real problem, but it could
seriously affect out of idle latencies.

You would really need a cheaper per CPU timer that doesn't
stop in idle (which rules out sched_clock). It's all unfortunately
quite messy.

-Andi
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