linux-kernel - Re: [rfc] lru_add_drain

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Message-Id: <20090910084602.9CBD.A69D9226@jp.fujitsu.com>
Date:	Thu, 10 Sep 2009 08:58:20 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	Christoph Lameter <cl@...ux-foundation.org>,
	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, Sep 9, 2009 at 1:27 PM, KOSAKI Motohiro
> <kosaki.motohiro@...fujitsu.com> wrote:
> >> The usefulness of a scheme like this requires:
> >>
> >> 1. There are cpus that continually execute user space code
> >>    without system interaction.
> >>
> >> 2. There are repeated VM activities that require page isolation /
> >>    migration.
> >>
> >> The first page isolation activity will then clear the lru caches of the
> >> processes doing number crunching in user space (and therefore the first
> >> isolation will still interrupt). The second and following isolation will
> >> then no longer interrupt the processes.
> >>
> >> 2. is rare. So the question is if the additional code in the LRU handling
> >> can be justified. If lru handling is not time sensitive then yes.
> >
> > 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.
> >
> > 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.
> 
> I think, more exactly, we don't have to drain lru pages for mlocking.
> Mlocked pages will go into unevictable lru due to
> try_to_unmap when shrink of lru happens.

Right.

> How about removing draining in case of mlock?

Umm, I don't like this. because perfectly no drain often make strange test result.
I mean /proc/meminfo::Mlock might be displayed unexpected value. it is not leak. it's only lazy cull.
but many tester and administrator wiill think it's bug... ;)

Practically, lru_add_drain_all() is nearly zero cost. because mlock's page fault is very
costly operation. it hide drain cost. now, we only want to treat corner case issue. 
I don't hope dramatic change.



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