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

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Message-ID: <20130508141103.GA24865@feng-snb>
Date:	Wed, 8 May 2013 22:11:03 +0800
From:	Feng Tang <feng.tang@...el.com>
To:	Pavel Machek <pavel@....cz>
Cc:	Ingo Molnar <mingo@...nel.org>,
	John Stultz <john.stultz@...aro.org>,
	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

Hi Pavel,

On Wed, May 08, 2013 at 12:55:42PM +0200, Pavel Machek wrote:
> Hi!
> 
> > > > 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.
> 
> Yes. But the clock for TSC is not being generated in CPU (right?) and
> AFAICT, the code said "if the CPU is new enough, assume TSC is good
> timesource". You need good clock for good timesource.
> 
> > > [...] 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...
> 
> AFAICT we normally use RTC/PIT during runtime.

This is not true. AFAIK, most modern X86 processors used by
Laptop/Desktop/Servers/Smartphone is not using PIT as the runtime clocksource.
If you check the Linux laptop/desktop you are using, most probably it is using
TSC as clocksource, less likely the "hpet", and not likey the "pit".

Thanks,
Feng
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