Re: [PATCH 0/3] x86: fix hang when AP bringup is too slow

From: Igor Mammedov
Date: Tue Mar 18 2014 - 15:27:59 EST

On Tue, 18 Mar 2014 08:21:19 -0400
Prarit Bhargava <prarit@xxxxxxxxxx> 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.

> 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();
+ }

> 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 */

> Apologies for the late review,
> P.

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