From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S933213AbcLMNoU (ORCPT <rfc822;w@1wt.eu>);
        Tue, 13 Dec 2016 08:44:20 -0500
Received: from bombadil.infradead.org ([198.137.202.9]:60883 "EHLO
        bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S932532AbcLMNoS (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Tue, 13 Dec 2016 08:44:18 -0500
Date: Tue, 13 Dec 2016 14:43:58 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
        Borislav Petkov <bp@alien8.de>,
        Bruce Schlobohm <bruce.schlobohm@intel.com>,
        Roland Scheidegger <rscheidegger_lists@hispeed.ch>,
        Kevin Stanton <kevin.b.stanton@intel.com>,
        Allen Hung <allen_hung@dell.com>
Subject: Re: [patch 2/2] x86/tsc: Force TSC_ADJUST register to value >= zero
Message-ID: <20161213134358.GV3207@twins.programming.kicks-ass.net>
References: <20161213131115.764824574@linutronix.de>
 <20161213131211.397588033@linutronix.de>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20161213131211.397588033@linutronix.de>
User-Agent: Mutt/1.5.23.1 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

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.