linux-kernel - Re: [patch 00/18] SMP: Boot and CPU hotplug refactoring

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Date:	Thu, 3 May 2012 11:41:45 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Tejun Heo <tj@...nel.org>,
	David Rientjes <rientjes@...gle.com>, venki@...gle.com
Subject: Re: [patch 00/18] SMP: Boot and CPU hotplug refactoring - Part 1

On Fri, 20 Apr 2012, Suresh Siddha wrote:
> On Fri, 2012-04-20 at 15:47 +0200, Thomas Gleixner wrote:
> > On Fri, 20 Apr 2012, Peter Zijlstra wrote:
> > 
> > > On Fri, 2012-04-20 at 13:05 +0000, Thomas Gleixner wrote:
> > > > This first part moves the idle thread management for non-boot cpus
> > > > into the core. fork_idle() is called in a workqueue as it is
> > > > implemented in a few architectures already. This is necessary when not
> > > > all cpus are brought up by the early boot code as otherwise we would
> > > > take a ref on the user task VM of the thread which brings the cpu up
> > > > via the sysfs interface. 
> > > 
> > > So I was thinking about this and I think we should make that kthreadd
> > > instead of a random workqueue thread due to all that cgroup crap. People
> > > are wanting to place all sorts of kernel threads in cgroups and I'm
> > > still arguing that kthreadd should not be allowed in cgroups.
> > 
> > So your fear is that the idle_thread will end up in some random cgroup
> > because some illdesigned user space code decided to stick kernel
> > threads into cgroups.
> > 
> > Can we please have some sanity restrictions on this cgroup muck? I
> > don't care when user space creates cgroups in circles, but holding the
> > whole kernel hostage of this madness is going too far.
> > 
> 
> Also, do we really need the workqueue/kthreadd based allocation? Just
> like the percpu areas getting allocated for each possible cpu during
> boot, shouldn't we extend this to the per-cpu idle threads too? So
> something like the appended should be ok to?

The idea is correct, there are just a few problems :)
 
> Signed-off-by: Suresh Siddha <suresh.b.siddha@...el.com>
> ---
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 05c46ba..a5144ab 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -303,10 +303,6 @@ static int __cpuinit _cpu_up(unsigned int cpu, int tasks_frozen)
>  
>  	cpu_hotplug_begin();
>  
> -	ret = smpboot_prepare(cpu);
> -	if (ret)
> -		goto out;
> -

If we failed to allocate an idle_thread for this cpu in smp_init()
then we unconditionally call __cpu_up() with a NULL pointer. That
might surprise the arch code :)

Aside of that, we now miss to reinitialize the idle thread. We call
init_idle() once when we allocate the thread, but not after a cpu
offline operation. That might leave stuff in weird state.

Thanks,

	tglx
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