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To: Venki Pallipadi <venkatesh.pallipadi@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
       Andrew Morton <akpm@linux-foundation.org>, Ingo Molnar <mingo@elte.hu>,
       linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] Track accurate idle time with tick_sched.idle_sleeptime
References: <20070827215611.GA30396@linux-os.sc.intel.com>
From: Andi Kleen <andi@firstfloor.org>
Date: 02 Sep 2007 00:23:53 +0200
In-Reply-To: <20070827215611.GA30396@linux-os.sc.intel.com>
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Venki Pallipadi <venkatesh.pallipadi@intel.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