linux-kernel - Re: perf hw in kexeced kernel broken in tip

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Date:	Wed, 01 Dec 2010 13:48:07 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Vivek Goyal <vgoyal@...hat.com>, Yinghai Lu <yinghai@...nel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Jason Wessel <jason.wessel@...driver.com>,
	Don Zickus <dzickus@...hat.com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	Haren Myneni <hbabu@...ibm.com>
Subject: Re: perf hw  in kexeced kernel broken in tip

Peter Zijlstra <peterz@...radead.org> writes:

> On Wed, 2010-12-01 at 14:58 -0500, Vivek Goyal wrote:
>> On Wed, Dec 01, 2010 at 08:49:49PM +0100, Peter Zijlstra wrote:
>> > On Wed, 2010-12-01 at 14:46 -0500, Vivek Goyal wrote:
>> > > On Wed, Dec 01, 2010 at 08:38:12PM +0100, Peter Zijlstra wrote:
>> > > > On Wed, 2010-12-01 at 11:23 -0500, Vivek Goyal wrote:
>> > > > > > What does kexec normally do to ensure hardware is left in a sane state?
>> > > > > 
>> > > > > Typically calls device_shutdown() and sysdev_shutdown() from
>> > > > > kernel_restart_prepare() to shutdown the devices.
>> > > > > 
>> > > > > Also calls machine_shutdown() which depending on architecture can take
>> > > > > care of various things like stopping other cpus, shutting down LAPIC, 
>> > > > > disabling IOAPIC, disabling hpet, shutting down IOMMU etc
>> > > > > (native_machine_shutdown()). 
>> > > > 
>> > > > So basically there's no sane generic reset callout?
>> > > 
>> > > I think ->shutdown() calls are sane generic callouts. Isn't it?
>> > 
>> > ->shutdown looks like it's about to reset/halt the hardware, no point in
>> > slowing down the regular shutdown/reboot path for something like this,
>> > we know the hardware will get reset to a sane state.
>> 
>> I think we already call ->shutdown() in regular reboot path.
>> 
>> kernel_restart()
>>   kernel_restart_prepare()
>>     device_shutdown();
>>     sysdev_shutdown();
>> 
>> So it should not make lot of difference if perf subsystem/counters are
>> also shutdown using ->shutdown().
>
> Oh, but I'm not a device or sysdev thing, I'll never get something like
> that.

There is also the reboot notifier, if the NMI needs to be controlled
outside of device model.  Sigh. The NMI handling is such a special case.

>> > > There seem to be few exceptions for LAPIC, IOMMU and HPET and I am not
>> > > sure why they are not covered by shutdown calls. CCing Eric, he might
>> > > have more insight into it.
>> > 
>> > That's all arch specific, but even there I don't think the reset code
>> > should live outside of kexec.
>> 
>> I would not know the history but I have heard stories that if you don't
>> shutdown the hardware over restart, BIOS might not be expecting it and
>> might get trumped.
>
> Never yet had a problem with that.

I haven't personally but I have certainly heard stories and seen
debugging sessions where some devices work or don't depending on the
order of running linux and windows on a machine, with soft reboots in
between.

Eric

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