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SubjectRe: [PATCH] memcg: use ID in page_cgroup
On Mon, 23 Aug 2010 14:32:37 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

> On Fri, 20 Aug 2010 19:01:32 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
>
> >
> > I have an idea to remove page_cgroup->page pointer, 8bytes reduction per page.
> > But it will be after this work.
> Another off topic. I think we can reduce the size of mem_cgroup by packing
> some boolean members into one "unsinged long flags".
>
I'll use "flags" to remove struct page pointer.


> > @@ -300,12 +300,13 @@ static atomic_t mem_cgroup_num;
> > #define NR_MEMCG_GROUPS (CONFIG_MEM_CGROUP_MAX_GROUPS + 1)
> > static struct mem_cgroup *mem_cgroups[NR_MEMCG_GROUPS] __read_mostly;
> >
> > -/* Must be called under rcu_read_lock */
> > -static struct mem_cgroup *id_to_memcg(unsigned short id)
> > +/* Must be called under rcu_read_lock, set safe==true if under lock */
> Do you mean, "Set safe==true if we can ensure by some locks that the id can be
> safely dereferenced without rcu_read_lock", right ?
>

yes. that's just for rcu_deerference_check().


> > +static struct mem_cgroup *id_to_memcg(unsigned short id, bool safe)
> > {
> > struct mem_cgroup *ret;
> > /* see mem_cgroup_free() */
> > - ret = rcu_dereference_check(mem_cgroups[id], rch_read_lock_held());
> > + ret = rcu_dereference_check(mem_cgroups[id],
> > + rch_read_lock_held() || safe);
> > if (likely(ret && ret->valid))
> > return ret;
> > return NULL;
>
> (snip)
> > @@ -723,6 +729,11 @@ static inline bool mem_cgroup_is_root(st
> > return (mem == root_mem_cgroup);
> > }
> >
> > +static inline bool mem_cgroup_is_rootid(unsigned short id)
> > +{
> > + return (id == 1);
> > +}
> > +
> It might be better to add
>
> BUG_ON(newid->id != 1)
>
> in cgroup.c::cgroup_init_idr().
>

Why ??

Thanks,
-Kame





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