On Thu, May 15, 2014 at 12:12:17PM +0530, Srivatsa S. Bhat wrote:
> 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 ?
Yeah looks good.
Thanks.
>
> >> + }
> >>
> >> llist_for_each_entry_safe(csd, csd_next, entry, llist) {
> >> csd->func(csd->info);
> >>
> >
>
> Regards,
> Srivatsa S. Bhat
>
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