Re: [PATCH v3 1/2] smp: Print more useful debug info upon receiving IPI on an offline CPU

[Frederic Weisbecker <fweisbec@gmail.com>]

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.bhat@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);
>