linux-kernel - Re: Fwd: [GIT PULL] timer changes for v3.10

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Message-ID: <20130508064419.GA5378@gmail.com>
Date:	Wed, 8 May 2013 08:44:19 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Pavel Machek <pavel@....cz>
Cc:	John Stultz <john.stultz@...aro.org>,
	Feng Tang <feng.tang@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Fwd: [GIT PULL] timer changes for v3.10

* Pavel Machek <pavel@....cz> wrote:

> > Sorry. You seem to not like the merged change, but I guess I'm not
> > quite sure what exactly your objection is here.
> 
> I'm not exactly sure what my objections are.
> 
> TSC was not designed for long-term precise timekeeping. [...]

The TSC is just a 64-bit counter that can be read very cheaply.

If the TSC is _implemented_ precisely in hardware and is kept in sync over 
CPUs then it's obviously fit for long-term precise timekeeping from that 
point on.

> [...] I guess it may work ok for short naps, [...]

Historically the TSC was not very precise nor kept in sync, but see the 
measurements from Feng Tang, it's very precise now on good hardware - and 
it's also a very cheap to read clocksource.

> [...] but some people suspend their machines for longer than that. Plus 
> I wonder how it will interfere with /etc/adjtime.

If it's precise then why should it interfere?

The history of the TSC being problematic can be ignored the moment CPU 
makers fix it completely - and apparently that is happening...

Thanks,

	Ingo
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