linux-kernel - Re: [RFC][PATCH] lru_add_drain_all() don't use schedule_on_each

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Message-Id: <1225037872.32713.22.camel@twins>
Date:	Sun, 26 Oct 2008 17:17:52 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	Nick Piggin <npiggin@...e.de>, linux-kernel@...r.kernel.org,
	Hugh Dickins <hugh@...itas.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Lee Schermerhorn <lee.schermerhorn@...com>, linux-mm@...ck.org,
	Christoph Lameter <cl@...ux-foundation.org>,
	Gautham Shenoy <ego@...ibm.com>,
	Oleg Nesterov <oleg@...sign.ru>,
	Rusty Russell <rusty@...tcorp.com.au>, mpm <mpm@...enic.com>
Subject: Re: [RFC][PATCH] lru_add_drain_all() don't use
 schedule_on_each_cpu()

On Mon, 2008-10-27 at 00:51 +0900, KOSAKI Motohiro wrote:
> >> >> @@ -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.
> 
> True. but I don't think it is big problem. because
> 
> 1. people can use grep filter easily.
> 2. that is just "sense of beauty" issue. not real pain.
> 3. current ps output is already utterly filled by kernel thread on
> large server :)
>     the patch doesn't introduce new problem.

Sure, its already bad, which is why I think we should see to it it
doesn't get worse - also we could make kthreads use CLONE_PID in which
case they'd all get collapsed, but that would be a use-visible change
which might up-set folks even more.

> > 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.
> 
> In my embedded job experience, I don't hear that.
> Their folks strongly interest to memory size and cpu usage, but don't
> interest # of thread so much.
> 
> Yes, too many thread spent many memory by stack. but the patch
> introduce only one thread on embedded device.

Right, and would be about 4k+sizeof(task_struct), some people might be
bothered, but most won't care.

> Perhaps, I misunderstand your intension. so can you point your
> previous discussion url?

my google skillz fail me, but once in a while people complain that we
have too many kernel threads.

Anyway, if we can re-use this per-cpu workqueue for more goals, I guess
there is even less of an objection.
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