On 05/13/2014 09:08 PM, Frederic Weisbecker wrote:
> On Mon, May 12, 2014 at 02:06:49AM +0530, Srivatsa S. Bhat wrote:
>> Today the smp-call-function code just prints a warning if we get an IPI on
>> an offline CPU. This info is sufficient to let us know that something went
>> wrong, but often it is very hard to debug exactly who sent the IPI and why,
>> from this info alone.
>>
>> In most cases, we get the warning about the IPI to an offline CPU,
>> immediately
>> after the CPU going offline comes out of the stop-machine phase and reenables
>> interrupts. Since all online CPUs participate in stop-machine, the
>> information
>> regarding the sender of the IPI is already lost by the time we exit the
>> stop-machine loop. So even if we dump the stack on each CPU at this point,
>> we won't find anything useful since all of them will show the stack-trace of
>> the stopper thread. So we need a better way to figure out who sent the IPI
>> and
>> why.
>>
>> To achieve this, when we detect an IPI targeted to an offline CPU, loop
>> through
>> the call-single-data linked list and print out the payload (i.e., the name
>> of the function which was supposed to be executed by the target CPU). This
>> would give us an insight as to who might have sent the IPI and help us debug
>> this further.
>>
>> Signed-off-by: Srivatsa S. Bhat <srivatsa.b...@linux.vnet.ibm.com>
>> ---
>>
>> kernel/smp.c | 18 ++++++++++++++----
>> 1 file changed, 14 insertions(+), 4 deletions(-)
>>
>> diff --git a/kernel/smp.c b/kernel/smp.c
>> index 06d574e..f864921 100644
>> --- a/kernel/smp.c
>> +++ b/kernel/smp.c
>> @@ -185,14 +185,24 @@ void generic_smp_call_function_single_interrupt(void)
>> {
>> struct llist_node *entry;
>> struct call_single_data *csd, *csd_next;
>> + static bool warned;
>> +
>> + entry = llist_del_all(&__get_cpu_var(call_single_queue));
>> + entry = llist_reverse_order(entry);
>>
>> /*
>> * Shouldn't receive this interrupt on a cpu that is not yet online.
>> */
>> - WARN_ON_ONCE(!cpu_online(smp_processor_id()));
>> -
>> - entry = llist_del_all(&__get_cpu_var(call_single_queue));
>> - entry = llist_reverse_order(entry);
>> + if (unlikely(!cpu_online(smp_processor_id()) && !warned)) {
>> + warned = true;
>> + WARN_ON(1);
>
> More details may be better:
>
> WARN_ONCE(1, "IPI on offline CPU");
>
Sure, that sounds better.
>> + /*
>> + * We don't have to use the _safe() variant here
>> + * because we are not invoking the IPI handlers yet.
>> + */
>> + llist_for_each_entry(csd, entry, llist)
>> + pr_warn("SMP IPI Payload: %pS \n", csd->func);
>
> Payload is kind of vague. How about "IPI func %pS sent on offline CPU".
>
Ok, and maybe s/func/function and s/on/to ?
>> + }
>>
>> llist_for_each_entry_safe(csd, csd_next, entry, llist) {
>> csd->func(csd->info);
>>
>
Regards,
Srivatsa S. Bhat
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