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Subject: Re: [RFC][PATCH] lru_add_drain_all() don't use
 schedule_on_each_cpu()
From: Peter Zijlstra <peterz@infradead.org>
To: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>, Nick Piggin <npiggin@suse.de>,
       linux-kernel@vger.kernel.org, Hugh Dickins <hugh@veritas.com>,
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Date: Sun, 26 Oct 2008 14:49:06 +0100
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On Sun, 2008-10-26 at 22:37 +0900, KOSAKI Motohiro wrote:
> Hi Peter,
> 
> >> @@ -611,4 +613,8 @@ void __init swap_setup(void)
> >>  #ifdef CONFIG_HOTPLUG_CPU
> >>       hotcpu_notifier(cpu_swap_callback, 0);
> >>  #endif
> >> +
> >> +     vm_wq = create_workqueue("vm_work");
> >> +     BUG_ON(!vm_wq);
> >> +
> >>  }
> >
> > While I really hate adding yet another per-cpu thread for this, I don't
> > see another way out atm.
> 
> Can I ask the reason of your hate?
> if I don't know it, making improvement patch is very difficult to me.

There seems to be no drive to keep them down, ps -def output it utterly
dominated by kernel threads on a freshly booted machine with many cpus.

And while they are not _that_ expensive to have around, they are not
free either, I imagine the tiny-linux folks having an interest in
keeping these down too.

> > Oleg, Rusty, ego, you lot were discussing a similar extra per-cpu
> > workqueue, can we merge these two?