linux-kernel - Re: [PATCH 2/4] stop_machine: reimplement using cpuhog

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Message-ID: <20100308171020.GC2557@osiris.boeblingen.de.ibm.com>
Date:	Mon, 8 Mar 2010 18:10:20 +0100
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org, rusty@...tcorp.com.au,
	sivanich@....com, torvalds@...ux-foundation.org, mingo@...e.hu,
	peterz@...radead.org, dipankar@...ibm.com, josh@...edesktop.org,
	paulmck@...ux.vnet.ibm.com, oleg@...hat.com,
	akpm@...ux-foundation.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH 2/4] stop_machine: reimplement using cpuhog

On Tue, Mar 09, 2010 at 12:53:21AM +0900, Tejun Heo wrote:
> Reimplement stop_machine using cpuhog.  As cpuhogs are guaranteed to
> be available for all online cpus, stop_machine_create/destroy() are no
> longer necessary and removed.
> 
> With resource management and synchronization handled by cpuhog, the
> new implementation is much simpler.  Asking the cpuhog to execute the
> stop_cpu() state machine on all online cpus with cpu hotplug disabled
> is enough.
> 
> stop_machine itself doesn't need to manage any global resources
> anymore, so all per-instance information is rolled into struct
> stop_machine_data and the mutex and all static data variables are
> removed.
> 
> The previous implementation created and destroyed RT workqueues as
> necessary which made stop_machine() calls highly expensive on very
> large machines.  According to Dimitri Sivanich, preventing the dynamic
> creation/destruction makes booting faster more than twice on very
> large machines.  cpuhog resources are preallocated for all online cpus
> and should have the same effect.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Cc: Rusty Russell <rusty@...tcorp.com.au>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Dimitri Sivanich <sivanich@....com>
> ---
>  arch/s390/kernel/time.c      |    1 -
>  drivers/xen/manage.c         |   14 +---
>  include/linux/stop_machine.h |   20 -----
>  kernel/cpu.c                 |    8 --
>  kernel/module.c              |   14 +---
>  kernel/stop_machine.c        |  162 ++++++++++--------------------------------
>  6 files changed, 42 insertions(+), 177 deletions(-)
> 
> diff --git a/arch/s390/kernel/time.c b/arch/s390/kernel/time.c
> index 65065ac..afe429e 100644
> --- a/arch/s390/kernel/time.c
> +++ b/arch/s390/kernel/time.c
> @@ -397,7 +397,6 @@ static void __init time_init_wq(void)
>  	if (time_sync_wq)
>  		return;
>  	time_sync_wq = create_singlethread_workqueue("timesync");
> -	stop_machine_create();
>  }
> 
>  /*

The reason we introduced stop_machine_create/destroy was to have a non-failing
variant that doesn't rely on I/O.
If we ever see a timesync machine check no I/O will succeed (it blocks) until
clocks have been synchronized. That means also that we rely on the non-blocking
semantics that those functions must have that are called via stop_machine.
This isn't true anymore with the cpu hog infrastructure:
if passed a blocking function that could wait on I/O we won't see any progress
anymore and the machine is dead.
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