linux-kernel - Re: [PATCH 2/2] mm: page allocator: Do not drain per-cpu lists via IPI from page allocator context

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Date:	Thu, 12 Jan 2012 16:08:04 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Gilad Ben-Yossef <gilad@...yossef.com>
Cc:	Mel Gorman <mgorman@...e.de>, Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Miklos Szeredi <mszeredi@...ell.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Greg KH <gregkh@...e.de>, Gong Chen <gong.chen@...el.com>
Subject: Re: [PATCH 2/2] mm: page allocator: Do not drain per-cpu lists via
 IPI from page allocator context

On Thu, 2012-01-12 at 16:51 +0200, Gilad Ben-Yossef wrote:
> What I can't figure out is why we don't need  get/put_online_cpus()
> pair around each and every call
> to on_each_cpu everywhere? and if we do, perhaps making it a part of
> on_each_cpu is the way to go?
> 
> Something like:
> 
> diff --git a/kernel/smp.c b/kernel/smp.c
> index f66a1b2..cfa3882 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -691,11 +691,15 @@ void on_each_cpu(void (*func) (void *info), void
> *info, int wait)
>  {
>         unsigned long flags;
> 
> +       BUG_ON(in_atomic());
> +
> +       get_online_cpus();
>         preempt_disable();

Your preempt_disable() here serializes against hotplug..

>         smp_call_function(func, info, wait);
>         local_irq_save(flags);
>         func(info);
>         local_irq_restore(flags);
>         preempt_enable();
> +       put_online_cpus();
>  }
>  EXPORT_SYMBOL(on_each_cpu); 
--
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