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Subject: Re: perf hw  in kexeced kernel broken in tip
From: Peter Zijlstra <peterz@infradead.org>
To: Vivek Goyal <vgoyal@redhat.com>
Cc: Yinghai Lu <yinghai@kernel.org>, Ingo Molnar <mingo@elte.hu>,
        Jason Wessel <jason.wessel@windriver.com>,
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        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Haren Myneni <hbabu@us.ibm.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>
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Date: Wed, 01 Dec 2010 21:07:16 +0100
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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 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.