Re: [PATCH 32/35] clockevents: Fix cpu down race for hrtimer based broadcasting

From: Preeti U Murthy <preeti@linux.vnet.ibm.com>
To: Nicolas Pitre <nicolas.pitre@linaro.org>,
	Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org,
	rjw@rjwysocki.net, tglx@linutronix.de
Subject: Re: [PATCH 32/35] clockevents: Fix cpu down race for hrtimer based broadcasting
Date: Thu, 26 Feb 2015 11:01:59 +0530	[thread overview]
Message-ID: <54EEAFCF.4080208@linux.vnet.ibm.com> (raw)
In-Reply-To: <alpine.LFD.2.11.1502231214240.25484@knanqh.ubzr>

On 02/23/2015 11:03 PM, Nicolas Pitre wrote:
> On Mon, 23 Feb 2015, Nicolas Pitre wrote:
> 
>> On Mon, 23 Feb 2015, Peter Zijlstra wrote:
>>
>>> The reported function that fails: bL_switcher_restore_cpus() is called
>>> in the error paths of the former and the main path in the latter to make
>>> the 'stolen' cpus re-appear.
>>>
>>> The patch in question somehow makes that go boom.
>>>
>>>
>>> Now what all do you need to do to make it go boom? Just enable/disable
>>> the switcher once and it'll explode? Or does it need to do actual
>>> switches while it is enabled?
>>
>> It gets automatically enabled during boot.  Then several switches are 
>> performed while user space is brought up.  If I manually disable it 
>> via /sys then it goes boom.
> 
> OK. Forget the bL switcher.  I configured it out of my kernel and then 
> managed to get the same crash by simply hotplugging out one CPU and 
> plugging it back in.
> 
> $ echo 0 > /sys/devices/system/cpu/cpu2/online
> [CPU2 gone]
> $ echo 1 > /sys/devices/system/cpu/cpu2/online
> [Boom!]
> 
> 
I saw an issue with this patch as well. I tried to do an smt mode switch
on a power machine, i.e varying the number of hyperthreads on an SMT 8
system, and the system hangs. Worse, there are no softlockup
messages/warnings/bug_ons reported. I am digging into this issue.

A couple of points though. Looking at the patch, I see that we are
shutting down tick device of the hotplugged out cpu *much before*
migrating the timers and hrtimers from it. Migration of timers is done
in the CPU_DEAD phase, while we shutdown tick devices in the CPU_DYING
phase. There is quite a bit of a gap here. Earlier we would do both in a
single notification.

Another point is that the tick devices are shutdown before the
hotplugged out cpu actually dies in __cpu_die(). At first look none of
these two points should create any issues. But since we are noticing
problems with this patch, I thought it would be best to put them forth.

But why are tick devices being shutdown that early ? Is there any
specific advantage to this? Taking/handing over tick duties should be
done before __cpu_die(), but shutdown of tick devices should be done
after this phase. This seems more natural, doesn't it?

Regards
Preeti U Murthy

> Nicolas
> 