Re: [rfc] lru_add_drain_all() vs isolation

[KOSAKI Motohiro]

> On Thu, 10 Sep 2009 08:58:20 +0900 (JST)
> KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> wrote:
> 
> > > On Wed, Sep 9, 2009 at 1:27 PM, KOSAKI Motohiro
> > > <kosaki.motohiro@xxxxxxxxxxxxxx> wrote:
> > > >> The usefulness of a scheme like this requires:
> > > >>
> > > >> 1. There are cpus that continually execute user space code
> > > >> Â Âwithout system interaction.
> > > >>
> > > >> 2. There are repeated VM activities that require page isolation /
> > > >> Â Âmigration.
> > > >>
> > > >> 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
> > > >
> > > > Â- if system isn't under heavy pressure, retry succussfull.
> > > > Â- 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... ;)
> 
> I agree. I have no objection to your approach. :)
> 
> > 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.
> 
> Another problem is as follow.
> 
> Although some CPUs don't have any thing to do, we do it. 
> HPC guys don't want to consume CPU cycle as Christoph pointed out.
> I liked Peter's idea with regard to this. 
> My approach can solve it, too. 
> But I agree it would be dramatic change. 

Is Perter's + mine approach bad?

It mean,

  - RT-thread binding cpu is not grabbing the page
	-> mlock successful by Peter's improvement
  - RT-thread binding cpu is grabbing the page
	-> mlock successful by mine approach
	   the page is culled later.




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