linux-kernel - Re: [PATCH 2/2] Use ktime_get() instead of sched

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Message-ID: <20100612071847.GA29751@elte.hu>
Date:	Sat, 12 Jun 2010 09:18:47 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Divyesh Shah <dpshah@...gle.com>
Cc:	jaxboe@...ionio.com, peterz@...radead.org, piotr@...owicz.com,
	linux-kernel@...r.kernel.org, vgoyal@...hat.com,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/2] Use ktime_get() instead of sched_clock() for blkio
 cgroup stats.


* Divyesh Shah <dpshah@...gle.com> wrote:

> This will take care of the pre-emptive kernel issue and the unbounded
> TSC drift problem. We will lose resolution though in some cases.

> -	blkg->stats.start_group_wait_time = sched_clock();
> +	blkg->stats.start_group_wait_time = ktime_to_ns(ktime_get());

Ugh!

ktime_get() can have insanely high overhead. Peter has added local_clock(), if 
then you should use that and apply checks to make sure the result isnt 
negative.

	Ingo
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