linux-kernel - Re: [PATCH] kernel/cpu.c: Move the CPU

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Message-ID: <20080831191721.GH7015@linux.vnet.ibm.com>
Date:	Sun, 31 Aug 2008 12:17:21 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	akpm@...ux-foundation.org
Subject: Re: [PATCH] kernel/cpu.c: Move the CPU_DYING notifiers

On Sun, Aug 31, 2008 at 07:58:49PM +0200, Manfred Spraul wrote:
> When a cpu is taken offline, the CPU_DYING notifiers are called on the
> dying cpu. According to <linux/notifiers.h>, the cpu should be "not
> running any task, not handling interrupts, soon dead".
> 
> For the current implementation, this is not true:
> - __cpu_disable can fail. If it fails, then the cpu will remain alive
>   and happy.
> - At least on x86, __cpu_disable() briefly enables the local interrupts
>   to handle any outstanding interrupts.
> 
> What about moving CPU_DYING down a few lines, behind the __cpu_disable()
> line?
> There are only two CPU_DYING handlers in the kernel right now: one in
> kvm, one in the scheduler. Both should work with the patch applied
> [and: I'm not sure if either one handles a failing __cpu_disable()]
> 
> The patch survives simple offlining a cpu. kvm untested due to lack
> of a test setup.

Several architectures re-enable interrupts in __cpu_disable() or in
functions called from __cpu_disable(), which happens after CPU_DYING,
if I understand correctly.  :-(

							Thanx, Paul

> Signed-Off-By: Manfred Spraul <manfred@...orfullife.com>
> ---
>  kernel/cpu.c |    5 +++--
>  1 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index e202a68..5b7c88f 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -199,13 +199,14 @@ static int __ref take_cpu_down(void *_param)
>  	struct take_cpu_down_param *param = _param;
>  	int err;
> 
> -	raw_notifier_call_chain(&cpu_chain, CPU_DYING | param->mod,
> -				param->hcpu);
>  	/* Ensure this CPU doesn't handle any more interrupts. */
>  	err = __cpu_disable();
>  	if (err < 0)
>  		return err;
> 
> +	raw_notifier_call_chain(&cpu_chain, CPU_DYING | param->mod,
> +				param->hcpu);
> +
>  	/* Force idle task to run as soon as we yield: it should
>  	   immediately notice cpu is offline and die quickly. */
>  	sched_idle_next();
> -- 
> 1.5.5.1
> 
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