Subject: Re: smp: Start up non-boot CPUs asynchronously

From: "Srivatsa S. Bhat" <[email-masked]>
Date: Tue, 14 Feb 2012 13:47:21 +0530

On 01/31/2012 09:54 PM, Arjan van de Ven wrote:

> From ee65be59057c920042747d46dc174c5a5a56c744 Mon Sep 17 00:00:00 2001
> From: Arjan van de Ven <[email-masked]>
> Date: Mon, 30 Jan 2012 20:44:51 -0800
> Subject: [PATCH] smp: Start up non-boot CPUs asynchronously
> 
> The starting of the "not first" CPUs actually takes a lot of boot time
> of the kernel... upto "minutes" on some of the bigger SGI boxes.
> Right now, this is a fully sequential operation with the rest of the kernel
> boot.
> 
> This patch turns this bringup of the other cpus into an asynchronous operation.
> With some other changes (not in this patch) this can save significant kernel
> boot time (upto 40% on my laptop!!).
> Basically now CPUs could get brought up in parallel to disk enumeration, graphic
> mode bringup etc etc etc.
> 
> Note that the implementation in this patch still waits for all CPUs to
> be brought up before starting userspace; I would love to remove that
> restriction over time (technically that is simple), but that becomes
> then a change in behavior... I'd like to see more discussion on that
> being a good idea before I write that patch.
> 
> Second note on version 2 of the patch:
> This patch does currently not save any boot time, due to a situation
> where the cpu hotplug lock gets taken for write by the cpu bringup code,
> which starves out readers of this lock throughout the kernel.
> Ingo specifically requested this behavior to expose this lock problem.
> 
> CC: Milton Miller <[email-masked]>
> CC: Andrew Morton <[email-masked]>
> CC: Ingo Molnar <[email-masked]>
> 
> Signed-off-by: Arjan van de Ven <[email-masked]>
> ---
>  kernel/smp.c |   21 ++++++++++++++++++++-
>  1 files changed, 20 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/smp.c b/kernel/smp.c
> index db197d6..ea48418 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -12,6 +12,8 @@
>  #include <linux/gfp.h>
>  #include <linux/smp.h>
>  #include <linux/cpu.h>
> +#include <linux/async.h>
> +#include <linux/delay.h>
> 
>  #ifdef CONFIG_USE_GENERIC_SMP_HELPERS
>  static struct {
> @@ -664,17 +666,34 @@ void __init setup_nr_cpu_ids(void)
>  	nr_cpu_ids = find_last_bit(cpumask_bits(cpu_possible_mask),NR_CPUS) + 1;
>  }
> 
> +void __init async_cpu_up(void *data, async_cookie_t cookie)
> +{
> +	unsigned long nr = (unsigned long) data;
> +	/*
> +	 * we can only up one cpu at a time, as enforced by the hotplug
> +	 * lock; it's better to wait for all earlier CPUs to be done before
> +	 * we bring up ours, so that the bring up order is predictable.
> +	 */
> +	async_synchronize_cookie(cookie);
> +	cpu_up(nr);
> +}
> +
>  /* Called by boot processor to activate the rest. */
>  void __init smp_init(void)
>  {
>  	unsigned int cpu;
> 
>  	/* FIXME: This should be done in userspace --RR */
> +
> +	/*
> +	 * But until we do this in userspace, we're going to do this
> +	 * in parallel to the rest of the kernel boot up.-- Arjan
> +	 */
>  	for_each_present_cpu(cpu) {
>  		if (num_online_cpus() >= setup_max_cpus)
>  			break;
>  		if (!cpu_online(cpu))
> -			cpu_up(cpu);
> +			async_schedule(async_cpu_up, (void *) cpu);
>  	}
> 
>  	/* Any cleanup work */

If I understand correctly, with this patch, the booting of non-boot CPUs
will happen in parallel with the rest of the kernel boot, but bringing up
of individual CPU is still serialized (due to hotplug lock).

If that is correct, I see several issues with this patch:

1. In smp_init(), after the comment "Any cleanup work" (see above), we print:
printk(KERN_INFO "Brought up %ld CPUs\n", (long)num_online_cpus());
So this can potentially print less than expected number of CPUs and might
surprise users.

2. Just below that we have smp_cpus_done(setup_max_cpus); and this translates
to native_smp_cpus_done() under x86, which calls impress_friends().
And that means, the bogosum calculation and the total activated processor
count which is printed, may get messed up.

3. sched_init_smp() is called immediately after smp_init(). And that calls
init_sched_domains(cpu_active_mask). Of course, it registers a hotplug
notifier callback to handle hot-added cpus.. but with this patch, boot up can
actually become unnecessarily slow at this point - what could have been done
in one go with an appropriately filled up cpu_active_mask, needs to be done
again and again using notifier callbacks. IOW, building sched domains can
potentially become a bottleneck, especially if there are lots and lots of
cpus in the machine.

4. There is an unhandled race condition (tiny window) in sched_init_smp():

get_online_cpus();
...
init_sched_domains(cpu_active_mask);
...
put_online_cpus();
                                           <<<<<<<<<<<<<<<<<<<<<<<< There!

hotcpu_notifier(cpuset_cpu_active, CPU_PRI_CPUSET_ACTIVE);
hotcpu_notifier(cpuset_cpu_inactive, CPU_PRI_CPUSET_INACTIVE);

At the point shown above, some non-boot cpus can get booted up, without
being noticed by the scheduler.

5. And in powerpc, it creates a new race condition, as explained in
https://lkml.org/lkml/2012/2/13/383
(Of course, we can fix it trivially by using get/put_online_cpus().)

There could be many more things that this patch breaks.. I haven't checked
thoroughly.

Regards,
Srivatsa S. Bhat