linux-kernel - Re: One-shot high-resolution POSIX timer periodically late

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Date:	Wed, 07 Feb 2007 11:13:58 +0100
From:	John <linux.kernel@...e.fr>
To:	linux-kernel@...r.kernel.org
CC:	mingo@...e.hu, tglx@...esys.com, johnstul@...ibm.com,
	akpm@...l.org, linux.kernel@...e.fr
Subject: Re: One-shot high-resolution POSIX timer periodically late

Ingo Molnar wrote:

> John wrote:
> 
>> Ingo Molnar wrote:
>> 
>>> 2.6.18-rt7 is pretty old, do you see the problem in 2.6.20-rc6-rt5 too?
>> 
>> Ingo, Thomas,
>> 
>> I cannot reproduce in 2.6.20-rc6-rt6, but it is not strictly an 
>> apples-to-apples comparison because I get a tsc clocksource in 
>> 2.6.20-rc6-rt6, and an acpi_pm clocksource in 2.6.18.6-rt7.
> 
> That suggests that the TSC was not deemed reliable by the newer kernel, 
> which almost always means it's not realiable. I think the problems you 
> were seeing are consistent with TSC being non-monotonic - so the fix is 
> to not use the TSC - and the newer kernel does this automatically.

Ingo,

Perhaps you've mis-read my message?

*2.6.18.6-rt7* installed an acpi_pm clocksource. When I used a boot 
parameter to force the clocksource to tsc, the high-resolution timer 
infrastructure broke (timers expired several *ms* too late).

On the other hand, *2.6.20-rc6-rt6* and *2.6.20-rt2* both installed a 
tsc clock source, and it works.

Therefore, it is not the newer kernel that ignored the tsc, but the 
older kernel. Thus I don't know what to make of your comment :-)

Regards,

John

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