linux-kernel - Re: [PATCH] kernel/watchdog: fix spurious hard lockups

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Date:   Wed, 21 Jun 2017 14:02:48 -0700
From:   Marc Herbert <Marc.Herbert@...el.com>
To:     Prarit Bhargava <prarit@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>
Cc:     kan.liang@...el.com, linux-kernel@...r.kernel.org,
        dzickus@...hat.com, mingo@...nel.org, akpm@...ux-foundation.org,
        babu.moger@...cle.com, atomlin@...hat.com,
        torvalds@...ux-foundation.org, peterz@...radead.org,
        tglx@...utronix.de, eranian@...gle.com, acme@...hat.com,
        stable@...r.kernel.org
Subject: Re: [PATCH] kernel/watchdog: fix spurious hard lockups

On 20/06/17 17:12, Prarit Bhargava wrote:
>>> Hmm ... odd that I haven't seen this.  We're running a pretty wide
>>> variety of systems here.  Do you have a reproducer?  I'd like to see
>>> this occur on production HW.

"Production" is where this patch was born and still lives right now:

https://chromium-review.googlesource.com/c/506327/ 

>> It only happens on a few specific CPU SKUs with a very wide Turbo range.
> 
> Which ones?
> 

The ones with turbo mode > 2.5 x TSC_MHz when you stress them hard and long
enough. Simple maths: just compare the soft and hard timers in the code.

The factor 3 moves the condition to: turbo > 7.5 x TSC_MHz.