Messages in this thread | | | Date | Wed, 07 May 2014 02:53:19 +0530 | From | "Srivatsa S. Bhat" <> | Subject | Re: [PATCH 1/2] smp: Print more useful debug info upon receiving IPI on an offline CPU |
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On 05/07/2014 02:04 AM, Andrew Morton wrote: > On Tue, 06 May 2014 23:32:51 +0530 "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com> 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. >> >> ... >> >> --- a/kernel/smp.c >> +++ b/kernel/smp.c >> @@ -185,15 +185,28 @@ void generic_smp_call_function_single_interrupt(void) >> { >> struct llist_node *entry; >> struct call_single_data *csd, *csd_next; >> + int warn = 0; >> >> /* >> * Shouldn't receive this interrupt on a cpu that is not yet online. >> */ >> - WARN_ON_ONCE(!cpu_online(smp_processor_id())); >> + if (unlikely(!cpu_online(smp_processor_id()))) { >> + warn = 1; >> + WARN_ON_ONCE(1); >> + } >> >> entry = llist_del_all(&__get_cpu_var(call_single_queue)); >> entry = llist_reverse_order(entry); >> >> + if (unlikely(warn)) { >> + /* >> + * 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); >> + } >> + > > This will emit the WARN_ON a single time, but will emit the "IPI > Payload" list every time the cpu is found to be offline. So on the > second and successive occurrences some output will still occur. > > Unfortunately WARN_ON_ONCE() returns the value of `condition', not > `__warned', so we have to hand-code things. Like this? >
Yeah, this version looks better. Sorry for missing this earlier. I'll incorporate this in my next version of the patchset.
Thanks a lot!
Regards, Srivatsa S. Bhat
> 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. > */ > if (unlikely(!cpu_online(smp_processor_id()) && !warned)) { > warned = true; > WARN_ON(1); > /* > * 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); > } > > llist_for_each_entry_safe(csd, csd_next, entry, llist) { > csd->func(csd->info); > csd_unlock(csd); > } > } > > > --- a/kernel/smp.c~smp-print-more-useful-debug-info-upon-receiving-ipi-on-an-offline-cpu-fix > +++ a/kernel/smp.c > @@ -185,20 +185,17 @@ void generic_smp_call_function_single_in > { > struct llist_node *entry; > struct call_single_data *csd, *csd_next; > - int warn = 0; > - > - /* > - * Shouldn't receive this interrupt on a cpu that is not yet online. > - */ > - if (unlikely(!cpu_online(smp_processor_id()))) { > - warn = 1; > - WARN_ON_ONCE(1); > - } > + static bool warned; > > entry = llist_del_all(&__get_cpu_var(call_single_queue)); > entry = llist_reverse_order(entry); > > - if (unlikely(warn)) { > + /* > + * Shouldn't receive this interrupt on a cpu that is not yet online. > + */ > + if (unlikely(!cpu_online(smp_processor_id()) && !warned)) { > + warned = true; > + WARN_ON(1); > /* > * We don't have to use the _safe() variant here > * because we are not invoking the IPI handlers yet. > _ >
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