From: "Liang, Kan" <kan.liang@intel.com>
To: Thomas Gleixner <tglx@linutronix.de>
CC: Don Zickus <dzickus@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "mingo@kernel.org" <mingo@kernel.org>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "babu.moger@oracle.com" <babu.moger@oracle.com>,
        "atomlin@redhat.com" <atomlin@redhat.com>,
        "prarit@redhat.com" <prarit@redhat.com>,
        "torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
        "peterz@infradead.org" <peterz@infradead.org>,
        "eranian@google.com" <eranian@google.com>,
        "acme@redhat.com" <acme@redhat.com>,
        "ak@linux.intel.com" <ak@linux.intel.com>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>
Subject: RE: [PATCH V2] kernel/watchdog: fix spurious hard lockups
Thread-Topic: [PATCH V2] kernel/watchdog: fix spurious hard lockups
Thread-Index: AQHS6pyX93nZMscYGUu+rlSjZMkkxKIvVm6AgAErMwCAARD/gIAAjbWAgABZxoCABJ2VAIABkHmAgB6zMiD//+HwAIAA0r+Q//+RnoCAAIguAA==
Date: Mon, 17 Jul 2017 14:46:53 +0000
Message-ID: <37D7C6CF3E00A74B8858931C1DB2F0775371D9AE@SHSMSX103.ccr.corp.intel.com>
References: <20170621144118.5939-1-kan.liang@intel.com>
 <alpine.DEB.2.20.1706212235550.2152@nanos>
 <20170622154450.2lua7fdmigcixldw@redhat.com>
 <alpine.DEB.2.20.1706230952190.2647@nanos>
 <20170623162907.l6inpxgztwwkeaoi@redhat.com>
 <alpine.DEB.2.20.1706232348020.2234@nanos>
 <20170626201927.3ak7fk3yvdzbb4ay@redhat.com>
 <20170627201249.ll34ecwhpme3vh2u@redhat.com>
 <37D7C6CF3E00A74B8858931C1DB2F0775371D43E@SHSMSX103.ccr.corp.intel.com>
 <alpine.DEB.2.20.1707170912370.2185@nanos>
 <37D7C6CF3E00A74B8858931C1DB2F0775371D8AA@SHSMSX103.ccr.corp.intel.com>
 <alpine.DEB.2.20.1707171503400.2185@nanos>
In-Reply-To: <alpine.DEB.2.20.1707171503400.2185@nanos>
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> On Mon, 17 Jul 2017, Liang, Kan wrote:
> > > That doesn't make sense. What's the exact test procedure?
> >
> > I don't know the exact test procedure. The test case is from our customer.
> > I only know that the test case makes calls into the x11 libs.
> 
> Sigh. This starts to be silly. You test something and have no idea what it does?

As I said, the test case is from our customer. They only share binaries with us.
Actually, it's more proper to call it test suite. It includes dozens of small test.
I just reproduced the issue and verified all the three patches in our lab.
Then I report it here as request immediately.
So I know little about the test case for now. 
I will share more when I learn more.
Sorry for that.

> 
> > > > According to our test, only patch 3 works well.
> > > > The other two patches will hang the system eventually.
> 
> Hang the system eventually? Does that mean that the system stops working
> and the watchdog does not catch the problem?


Right, the system stops working and the watchdog does not catch the problem.

> 
> > > > BTW: We set 1 to watchdog_thresh when we did the test.
> > > > It's believed that can speed up the failure.
> > >
> > > Believe is not really a technical measure....
> > >
> >
> > 1 is a valid value for watchdog_thresh.
> > It was set through the standard proc interface.
> > /proc/sys/kernel/watchdog_thresh
> > It should not impacts the final test result.
> 
> I know that 1 is a valid value and I know how that can be set. Still, it does not
> help if you believe that setting the threshold to 1 can speed up the failure.
> Either you know it for sure or not. You can believe in god or whatever, but
> here we talk about facts.

I personally didn't compare the difference between 1 and default 10 for this
test case.
Before we had the test case from customer, we developed other micro
which can reproduce the similar issue.
For that micro, 1 can speed up the failure.
(BTW: all the three patches can fix the issue which was reproduced by that micro.)

If you think it's meaningful to verify 10 as well, I can do the compare.

Thanks,
Kan