linux-kernel - Re: [PATCH] x86: Document the hotplug code is incompatible with x86 irq handling

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Message-Id: <200706130016.09490.rjw@sisk.pl>
Date:	Wed, 13 Jun 2007 00:16:08 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pavel Machek <pavel@....cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <ak@...e.de>, linux-kernel@...r.kernel.org,
	Neil Brown <neilb@...e.de>, Ingo Molnar <mingo@...e.hu>,
	asit.k.mallick@...el.com
Subject: Re: [PATCH] x86: Document the hotplug code is incompatible with x86 irq handling

On Tuesday, 12 June 2007 23:56, Siddha, Suresh B wrote:
> On Tue, Jun 12, 2007 at 10:52:09PM +0200, Rafael J. Wysocki wrote:
> > On Tuesday, 12 June 2007 20:19, Eric W. Biederman wrote:
> > > Because you are calling unfixably broken code.  That should be a decent
> > > incentive to do something else won't it?
> > 
> > Can you please tell me _what_ else can be done?
> > 
> > > IOAPICs do not support what the code is doing here.  There is lots of
> > > practical evidence including bad experiences and practical tests that
> > > support this.
> > 
> > Well, AFAICS, Suresh has tried to debug one failing case recently without
> > any consistent conclusions.  I don't know of any other test cases (links,
> > please?).
> 
> Rafael, Darrick Wong's issue looks different and hence I was motivated to
> look and fix if it was a SW issue. For now, I am not able to comprehend
> what is happening on Darrick Wong's system. Need more help from Darrick
> as he has the golden failing system.
> 
> Meanwhile I talked to our hardware folks about the irq migration in general.
> 
> One good news is that future versions of chipsets will have an Interrupt
> Remapping feature(for more details please refer to
> http://download.intel.com/technology/computing/vptech/Intel(r)_VT_for_Direct_IO.pdf)
> where in we can reliably migrate the irq to someother cpu in the process
> context.
> 
> And for the existing platforms, chipset folks don't see a reason why the
> Eric's algorithm(specified below) should fail.
> 
> Eric's algorithm for level triggered(edge triggered should be easier than
> level triggered):
> - Mask the irq in process context.
> - Poll RIRR until an ack of for the irq was not pending.
> - Migrate the irq.
> 
> Eric had a problem of stuck remote IRR on E75xx chipset with this algorithm
> and my next step is to reproduce this issue on this platform and understand
> the behavior.

OK

In that case, do I understand correctly that we are going to implement the
Eric's algorithm above for the CPU hotunplugging on x86 once you've figured
out what's the E75xx issue?

Rafael


-- 
"Premature optimization is the root of all evil." - Donald Knuth
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