linux-kernel - Re: [PATCH v10 1/3] smpboot: allow excluding cpus from the smpboot threads

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Date:	Mon, 4 May 2015 18:06:24 -0400
From:	Chris Metcalf <cmetcalf@...hip.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Don Zickus <dzickus@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Jones <drjones@...hat.com>,
	Ulrich Obergfell <uobergfe@...hat.com>,
	Fabian Frederick <fabf@...net.be>,
	Aaron Tomlin <atomlin@...hat.com>,
	Ben Zhang <benzh@...omium.org>,
	Christoph Lameter <cl@...ux.com>,
	Gilad Ben-Yossef <gilad@...yossef.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	<linux-kernel@...r.kernel.org>, Jonathan Corbet <corbet@....net>,
	<linux-doc@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v10 1/3] smpboot: allow excluding cpus from the smpboot
 threads

On 5/1/2015 5:23 PM, Frederic Weisbecker wrote:
> On Fri, May 01, 2015 at 03:57:51PM -0400, Chris Metcalf wrote:
>
>> For example, booting with only cpu 0 as a housekeeping core (and
>> therefore all watchdogs 1-35 on my 36-core tilegx are parked), and
>> immediately doing "echo 0 > /proc/sys/kernel/watchdog", I see
>> (via SysRq ^O-l) the first parked watchdog, on cpu 1, hung with:
>>
>>    frame 0: 0xfffffff7000f2928 lock_hrtimer_base+0xb8/0xc0
>>    frame 1: 0xfffffff7000f2a28 hrtimer_try_to_cancel+0x40/0x170
>>    frame 2: 0xfffffff7000f2a28 hrtimer_try_to_cancel+0x40/0x170
>>    frame 3: 0xfffffff7000f2b98 hrtimer_cancel+0x40/0x68
>>    frame 4: 0xfffffff70014cce0 watchdog_disable+0x50/0x70
>>    frame 5: 0xfffffff70008c2d0 smpboot_thread_fn+0x350/0x438
>>    frame 6: 0xfffffff700084b28 kthread+0x160/0x178
> Have you tried to do that before your patchset?

Yes, it works fine.  It requires the presence of the parked threads to trigger the issue.

>> The config does not have NO_HZ_FULL_ALL or NO_HZ_FULL_SYSIDLE
>> set, and does have RCU_FAST_NO_HZ and RCU_NOCB_CPU_ALL.
>>
>> I don't really know how to start debugging this, but I do know that
>> unparking the threads first avoids the issue :-)
> Do you have CONFIG_PROVE_LOCKING=y ?

There seems to be some skew between the community version, which is throwing a
bunch of errors when I enable PROVE_LOCKING, and our internal version where some
things are not yet upstreamed but PROVE_LOCKING works :-)

I'll try to set aside some time to reconcile the two to figure it out.

-- 
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com

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