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

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Date:	Tue, 18 Mar 2014 08:21:19 -0400
From:	Prarit Bhargava <prarit@...hat.com>
To:	Igor Mammedov <imammedo@...hat.com>
CC:	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, bp@...e.de, paul.gortmaker@...driver.com,
	JBeulich@...e.com, drjones@...hat.com, toshi.kani@...com,
	x86@...nel.org, riel@...hat.com, gong.chen@...ux.intel.com
Subject: Re: [PATCH 0/3] x86: fix hang when AP bringup is too slow

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 ;)

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().

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. */ ?

Apologies for the late review,

P.
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