From: Michael Ellerman <michael@ellerman•id.au>
To: Brian King <brking@linux•vnet.ibm.com>
Cc: Michael Neuling <mikey@neuling•org>, linuxppc-dev@lists•ozlabs.org
Subject: Re: [PATCH 1/1] powerpc: Ignore IPIs to offline CPUs
Date: Wed, 21 Apr 2010 23:35:29 +1000 [thread overview]
Message-ID: <1271856929.3832.46.camel@concordia> (raw)
In-Reply-To: <4BCE6DDC.4020902@linux.vnet.ibm.com>
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On Tue, 2010-04-20 at 22:15 -0500, Brian King wrote:
> On 04/20/2010 09:04 PM, Michael Neuling wrote:
> > In message <201004210154.o3L1sXaR001791@d01av04•pok.ibm.com> you wrote:
> >>
> >> Since there is nothing to stop an IPI from occurring to an
> >> offline CPU, rather than printing a warning to the logs,
> >> just ignore the IPI. This was seen while stress testing
> >> SMT enable/disable.
> >
> > This seems like a recipe for disaster. Do we at least need a
> > WARN_ON_ONCE?
>
> Actually we are only seeing it once per offlining of a CPU,
> and only once in a while.
>
> My guess is that once the CPU is marked offline fewer IPIs
> get sent to it since its no longer in the online mask.
Hmm, right. Once it's offline it shouldn't get _any_ IPIs, AFAICS.
> Perhaps we should be disabling IPIs to offline CPUs instead?
You mean not sending them? We do:
void smp_xics_message_pass(int target, int msg)
{
unsigned int i;
if (target < NR_CPUS) {
smp_xics_do_message(target, msg);
} else {
for_each_online_cpu(i) {
if (target == MSG_ALL_BUT_SELF
&& i == smp_processor_id())
continue;
smp_xics_do_message(i, msg);
}
}
}
So it does sound like the IPI was sent while the cpu was online (ie.
before pseries_cpu_disable(), but xics_migrate_irqs_away() has not
caused the IPI to be cancelled.
Problem is I don't think we can just ignore the IPI. The IPI might have
been sent for a smp_call_function() which is waiting for the result, in
which case if we ignore it the caller will block for ever.
I don't see how to fix it :/
cheers
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next prev parent reply other threads:[~2010-04-21 13:35 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-04-21 1:54 [PATCH 1/1] powerpc: Ignore IPIs to offline CPUs Brian King
2010-04-21 2:04 ` Michael Neuling
2010-04-21 3:15 ` Brian King
2010-04-21 13:35 ` Michael Ellerman [this message]
2010-04-21 13:50 ` Brian King
2010-04-21 21:03 ` Michael Neuling
2010-04-21 22:15 ` Brian King
2010-04-21 22:49 ` Michael Neuling
2010-04-21 23:33 ` Brian King
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