On 04/21/2010 04:03 PM, Michael Neuling wrote:
> In message <4bcf029b.1020...@linux.vnet.ibm.com> you wrote:
>> On 04/21/2010 08:35 AM, Michael Ellerman wrote:
>>> 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.o3l1sxar001...@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 :/
>>
>> Any objections to just removing the warning?
>
> Well someone could be waiting for the result, so it could be a real
> problem.
>
> IMHO the warning should stay.
Looking in arch/powerpc/kernel/smp.c, there are four possible IPIs:
void smp_message_recv(int msg)
{
switch(msg) {
case PPC_MSG_CALL_FUNCTION:
generic_smp_call_function_interrupt();
break;
case PPC_MSG_RESCHEDULE:
/* we notice need_resched on exit */
break;
case PPC_MSG_CALL_FUNC_SINGLE:
generic_smp_call_function_single_interrupt();
break;
case PPC_MSG_DEBUGGER_BREAK:
if (crash_ipi_function_ptr) {
crash_ipi_function_ptr(get_irq_regs());
break;
}
#ifdef CONFIG_DEBUGGER
debugger_ipi(get_irq_regs());
break;
#endif /* CONFIG_DEBUGGER */
/* FALLTHROUGH */
Both generic_smp_call_function_interrupt and
generic_smp_call_function_single_interrupt
have WARN_ON(!cpu_online(cpu)); in them. The debugger IPI, appears to ignore
the IPI
if the cpu is offline, which leaves the reschedule IPI. This is likely the one
I am
seeing in test, since I'm not seeing the other WARN_ON's.
-Brian
--
Brian King
Linux on Power Virtualization
IBM Linux Technology Center
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