linux-kernel - Re: [tip:timers/core] clocksource: Resolve cpu hotplug dead lock with TSC unstable

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Message-ID: <alpine.LFD.2.00.0908311754590.19335@localhost.localdomain>
Date:	Mon, 31 Aug 2009 17:59:00 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
cc:	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	johnstul@...ibm.com, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:timers/core] clocksource: Resolve cpu hotplug dead lock
 with TSC unstable

On Mon, 31 Aug 2009, Martin Schwidefsky wrote:
> Ah, very good. I've been going round in circles to find a solution that
> allows to  downgrade the tsc rating when the second cpu is enabled.
> Could not find a solution. Your approach changes semantics slightly:
> the tsc clock will continue with its old rating for a while until the
> watchdog will do the downgrade. If that is acceptable then this is a
> good solution.

Even the old code did not switch right away. The TSC was used until
the next timer interrupt. Also we survive the detection of unstable
TSCs with the watchdog which can happen quite after the fact that it
became unusable. The important point is that we detect it at all and
replace it with something useable.

Thanks,

	tglx
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