Subject: Re: [PATCH] kernel/watchdog: fix spurious hard lockups
To: Prarit Bhargava <prarit@redhat.com>, Andi Kleen <ak@linux.intel.com>
References: <20170620213309.30051-1-kan.liang@intel.com>
 <4718a252-9515-626e-a69f-565f1c2bc589@redhat.com>
 <20170620230002.GE23705@tassilo.jf.intel.com>
 <9320cd00-88f4-49c5-aaa5-4bb4a80c8813@redhat.com>
Cc: kan.liang@intel.com, linux-kernel@vger.kernel.org, dzickus@redhat.com,
        mingo@kernel.org, akpm@linux-foundation.org, babu.moger@oracle.com,
        atomlin@redhat.com, torvalds@linux-foundation.org,
        peterz@infradead.org, tglx@linutronix.de, eranian@google.com,
        acme@redhat.com, stable@vger.kernel.org
From: Marc Herbert <Marc.Herbert@intel.com>
Message-ID: <6920d83c-19b9-9243-c6aa-0f21288a4006@intel.com>
Date: Wed, 21 Jun 2017 14:02:48 -0700
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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.