linux-kernel - Re: [rfc] lru_add_drain

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Date:	Thu, 10 Sep 2009 08:43:56 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	kosaki.motohiro@...fujitsu.com,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>, Ingo Molnar <mingo@...e.hu>,
	linux-mm <linux-mm@...ck.org>,
	Oleg Nesterov <onestero@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [rfc] lru_add_drain_all() vs isolation

> On Wed, 9 Sep 2009, KOSAKI Motohiro wrote:
> 
> > Christoph, I'd like to discuss a bit related (and almost unrelated) thing.
> > I think page migration don't need lru_add_drain_all() as synchronous, because
> > page migration have 10 times retry.
> 
> True this is only an optimization that increases the chance of isolation
> being successful. You dont need draining at all.
> 
> > Then asynchronous lru_add_drain_all() cause
> >
> >   - if system isn't under heavy pressure, retry succussfull.
> >   - if system is under heavy pressure or RT-thread work busy busy loop, retry failure.
> >
> > I don't think this is problematic bahavior. Also, mlock can use asynchrounous lru drain.
> >
> > What do you think?
> 
> The retries can be very fast if the migrate pages list is small. The
> migrate attempts may be finished before the IPI can be processed by the
> other cpus.

Ah, I see. Yes, my last proposal is not good. small migration might be fail.

How about this?
  - pass 1-2,  lru_add_drain_all_async()
  - pass 3-10, lru_add_drain_all()

this scheme might save RT-thread case and never cause regression. (I think)

The last remain problem is, if RT-thread binding cpu's pagevec has migrate
targetted page, migration still face the same issue.
but we can't solve it...
RT-thread must use /proc/sys/vm/drop_caches properly.



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