Messages in this thread | | | Date | Thu, 15 May 2014 12:12:17 +0530 | From | "Srivatsa S. Bhat" <> | Subject | Re: [PATCH v3 1/2] smp: Print more useful debug info upon receiving IPI on an offline CPU |
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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.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"); >
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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