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

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Message-ID: <AANLkTinXZ3lHxTcusKxcPG-FWvykcfiDB9lcpZIPLEKx@mail.gmail.com>
Date:	Sat, 12 Jun 2010 08:51:54 -0700
From:	Divyesh Shah <dpshah@...gle.com>
To:	Ingo Molnar <mingo@...e.hu>
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.

On Sat, Jun 12, 2010 at 12:18 AM, Ingo Molnar <mingo@...e.hu> wrote:
>
> * 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.

Ok. The negative checks are already in the code.

>
>        Ingo
>
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