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SubjectRe: [PATCH 0/3] x86: fix hang when AP bringup is too slow
On Wed, 19 Mar 2014 07:51:05 -0400
Prarit Bhargava <prarit@redhat.com> wrote:

>
>
> On 03/18/2014 02:49 PM, Igor Mammedov wrote:
> > On Tue, 18 Mar 2014 08:21:19 -0400
> > Prarit Bhargava <prarit@redhat.com> wrote:
> >
> >>
> >>
> >> On 03/13/2014 10:25 AM, Igor Mammedov wrote:
> >>> Hang is observed on virtual machines during CPU hotplug,
> >>> especially in big guests with many CPUs. (It happens more
> >>> often if host is over-committed).
> >>>
> >>
> >> Hey Igor, I like this better than the previous version. Thanks for taking into
> >> account the possible races in this code.
> >>
> >> A quick question on system behaviour. As you know I've been more concerned
> >> lately with error handling, etc., through the cpu hotplug code as we've seen
> >> several customer reports of silent failures or cascading failures in the cpu
> >> hotplug code when users have been attempting to perform physical hotplug.
> >>
> >> After your patches have been applied, in theory the following can happen:
> >>
> >> The master CPU is completing the AP cpu's bring up. The AP cpu is doing (sorry
> >> for the cut-and-paste),
> >>
> >> void cpu_init(void)
> >> {
> >> int cpu = smp_processor_id();
> >> struct task_struct *curr = current;
> >> struct tss_struct *t = &per_cpu(init_tss, cpu);
> >> struct thread_struct *thread = &curr->thread;
> >>
> >> /*
> >> * wait till the master CPU completes it's STARTUP sequence,
> >> * and decides to wait till this AP boots
> >> */
> >> while (!cpumask_test_cpu(cpu, cpu_callout_mask)) {
> >> cpu_relax();
> >> if (per_cpu(x86_cpu_to_apicid, cpu) == BAD_APICID)
> >> halt();
> >> }
> >>
> >> and is spinning on cpu_relax(). Suppose something goes wrong and the softlockup
> >> watchdog fires on the AP cpu:
> >>
> >> 1. Can it? :) ie) will the softlockup fire at this point of the AP init? Okay,
> >> I'm being really lazy and not looking at the code ;)
> > It shouldn't, CPU is in pristine state and just came from boot trampoline at
> > this point without interrupts configured yet.
>
> Okay, not a big problem.
>
> >
> >>
> >> 2. Is there anything we can do in this code to notify the user of a problem?
> >> Even a pr_crit() here I think would help to indicate what went wrong; it might
> >> be useful for future debugging in this area to have some sort of output. I
> >> think a WARN() or BUG() is necessary here as there are several calls to cpu_init().
> > Do you mean something like this:
> >
> > + if (per_cpu(x86_cpu_to_apicid, cpu) == BAD_APICID) {
> > + WARN(1);
> > + halt();
> > + }
>
> Yeah, maybe WARN_ON(1, "some comment") though.
printk at so early stage might be cause issues, since it is quite complex.
Its' disabling/enabling irqs, calls *_delay_*() functions and takes locks.
The last is especially dangerous because if AP is shot down by another
INIT/SIPI, system will hang on next printk if locks were acquired by AP
at that time.
That case is possible if master CPU has got errors during wakeup_ap() and
failed cpu_up() then it was unplugged + plugged via ACPI and attempted
to be onlined again.

It's much safer not to do anything complex at AP start-up so early.

BTW:
when AP reaches halt() line, failure is not silent. the master CPU might
print error message if debug level logging is active:
see arch/x86/kernel/smpboot.c:native_cpu_up()
...
if (err) {
pr_debug("do_boot_cpu failed %d\n", err);
return -EIO;
}
...

perhaps we should change pr_debug to pr_crit here to make it more visible.
something like:

@@ -858,7 +858,7 @@ int native_cpu_up(unsigned int cpu, struct task_struct *tidle)

err = do_boot_cpu(apicid, cpu, tidle);
if (err) {
- pr_debug("do_boot_cpu failed %d\n", err);
+ pr_crit("do_boot_cpu failed(%d) to wakeup CPU#%u\n", err, cpu);
return -EIO;
}

>
> >
> >>
> >> 3. Change this comment:
> >>
> >> * wait till the master CPU completes it's STARTUP sequence,
> >> * and decides to wait till this AP boots
> >>
> >> to
> >>
> >> /* wait for the master CPU to complete this cpu's STARTUP. */ ?
> > well, that is not quite the same as above, comment should underline that
> > AP waits for ACK from master CPU before continuing with this AP initialization.
> >
> > How about:
> > /* wait for ACK from master CPU before continuing with AP initialization */
>
> Awesome :)
>
> P.
>
> >
> >>
> >> Apologies for the late review,
> >>
> >> P.
> >
> >



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