linux-kernel - Re: [patch 2/2] x86/tsc: Force TSC_ADJUST register to value >= zero

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Message-ID: <20161213134358.GV3207@twins.programming.kicks-ass.net>
Date:   Tue, 13 Dec 2016 14:43:58 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>,
        Bruce Schlobohm <bruce.schlobohm@...el.com>,
        Roland Scheidegger <rscheidegger_lists@...peed.ch>,
        Kevin Stanton <kevin.b.stanton@...el.com>,
        Allen Hung <allen_hung@...l.com>
Subject: Re: [patch 2/2] x86/tsc: Force TSC_ADJUST register to value >= zero

On Tue, Dec 13, 2016 at 01:14:17PM -0000, Thomas Gleixner wrote:
> @@ -429,8 +447,13 @@ void check_tsc_sync_target(void)
>  	 * that the warp is not longer detectable when the observed warp
>  	 * value is used. In the worst case the adjustment needs to go
>  	 * through a 3rd run for fine tuning.
> +	 *
> +	 * But we must make sure that the value doesn't become negative
> +	 * otherwise TSC deadline timer will create an interrupt storm.
>  	 */
>  	cur->adjusted += cur_max_warp;
> +	if (cur->adjusted < 0)
> +		cur->adjusted = 0;
>  

So depending on how horrid we want to make this we could work around
this by increasing the ADJUST of the other CPUs by the amount we're
negative.

But yes, yuck.