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

[Ryo Tsuruta]

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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