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");
> + /*
> + * 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".
> + }
>
> llist_for_each_entry_safe(csd, csd_next, entry, llist) {
> csd->func(csd->info);
>
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