linux-kernel - Re: [PATCH 3/9] bio-cgroup controller

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Date:	Thu, 16 Apr 2009 21:00:02 +0900 (JST)
From:	Ryo Tsuruta <ryov@...inux.co.jp>
To:	righi.andrea@...il.com
Cc:	kamezawa.hiroyu@...fujitsu.com, menage@...gle.com,
	balbir@...ux.vnet.ibm.com, guijianfeng@...fujitsu.com,
	agk@...rceware.org, akpm@...ux-foundation.org, axboe@...nel.dk,
	baramsori72@...il.com, chlunde@...g.uio.no,
	dave@...ux.vnet.ibm.com, dpshah@...gle.com, eric.rannaud@...il.com,
	fernando@....ntt.co.jp, taka@...inux.co.jp, lizf@...fujitsu.com,
	matt@...ehost.com, dradford@...ehost.com, ngupta@...gle.com,
	randy.dunlap@...cle.com, roberto@...it.it, s-uchida@...jp.nec.com,
	subrata@...ux.vnet.ibm.com, yoshikawa.takuya@....ntt.co.jp,
	containers@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/9] bio-cgroup controller

Hi Andrea and Kamezawa-san,

> > > +#ifdef CONFIG_CGROUP_BIO
> > > +/*
> > > + * use lower 16 bits for flags and reserve the rest for the bio-cgroup id
> > > + */
> > > +#define BIO_CGROUP_ID_SHIFT	(16)
> > > +#define BIO_CGROUP_ID_BITS (8 * sizeof(unsigned long) - BIO_CGROUP_ID_SHIFT)
> > > +
> > > +static inline unsigned long page_cgroup_get_bio_id(struct page_cgroup *pc)
> > > +{
> > > +	return pc->flags >> BIO_CGROUP_ID_SHIFT;
> > > +}
> > > +
> > > +static inline void page_cgroup_set_bio_id(struct page_cgroup *pc,
> > > +				unsigned long id)
> > > +{
> > > +	WARN_ON(id >= (1UL << BIO_CGROUP_ID_BITS));
> > > +	pc->flags &= (1UL << BIO_CGROUP_ID_SHIFT) - 1;
> > > +	pc->flags |= (unsigned long)(id << BIO_CGROUP_ID_SHIFT);
> > > +}
> > > +#endif
> > > +
> > Ah, there is "Lock" bit in pc->flags and above "set" code does read-modify-write
> > without lock_page_cgroup().
> > 
> > Could you use lock_page_cgroup() or cmpxchg ? (or using something magical technique ?)
> 
> If I'm not wrong this should guarantee atomicity without using
> lock_page_cgroup().

I'll consider carefully how is the best way to minimize the overhead
as far as possible.
First, I'll post the new bio-cgroup patches that use css_id as
bio_cgroup_id soon.

Thanks,
Ryo Tsuruta
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