linux-kernel - Re: Question: Takes about 2s in sched_clock

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Message-ID: <5271F4E9.8010709@huawei.com>
Date:	Thu, 31 Oct 2013 14:12:57 +0800
From:	Xie XiuQi <xiexiuqi@...wei.com>
To:	Younger Liu <younger.liucn@...il.com>
CC:	kernel <linux-kernel@...r.kernel.org>
Subject: Re: Question: Takes about 2s in sched_clock_init() when kernel boots

On 2013/10/31 13:29, Younger Liu wrote:
> Hi,
>   When I analyse boot time on ArmV7, I found that it takes
> about 2s in sched_clock_init(), the log is as follow:
> [    0.000000] before:sched_clock_init.
> [    0.000000] start:sched_clock_init.
> [    2.100505] end  :sched_clock_init.
> [    2.103983] before:calibrate_delay.
> 
>   In my config, CONFIG_HAVE_UNSTABLE_SCHED_CLOCK does not config.
> 
>   Are there any suggestions?

see below:
u64 sched_clock_cpu(int cpu)
{
	struct sched_clock_data *scd;
	u64 clock;

	WARN_ON_ONCE(!irqs_disabled());

	if (sched_clock_stable)
		return sched_clock();

	if (unlikely(!sched_clock_running))
		return 0ull;

	scd = cpu_sdc(cpu);
[...]

Unless sched_clock_running is set, it'll return 0. So, the
printk() time stamp is 0 before sched_clock_init calling.

> 
> 	Younger
> --
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