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

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Message-ID: <20101201195835.GE2511@redhat.com>
Date:	Wed, 1 Dec 2010 14:58:35 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Yinghai Lu <yinghai@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Jason Wessel <jason.wessel@...driver.com>,
	Don Zickus <dzickus@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Haren Myneni <hbabu@...ibm.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: perf hw  in kexeced kernel broken in tip

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().

> 
> > 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.

Thanks
Vivek
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