Re: [PATCH -mm 2/5] use ID in page cgroup

[KAMEZAWA Hiroyuki]

On Tue, 3 Aug 2010 09:15:13 +0530
Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote:

> * KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> [2010-08-02 19:14:10]:
> 
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> > 
> > Now, addresses of memory cgroup can be calculated by their ID without complex.
> > This patch relplaces pc->mem_cgroup from a pointer to a unsigned short.
> > On 64bit architecture, this offers us more 6bytes room per page_cgroup.
> > Use 2bytes for blkio-cgroup's page tracking. More 4bytes will be used for
> > some light-weight concurrent access.
> > 
> > We may able to move this id onto flags field but ...go step by step.
> > 
> > Changelog: 20100730
> >  - fixed some garbage added by debug code in early stage
> > 
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> > ---
> >  include/linux/page_cgroup.h |    3 ++-
> >  mm/memcontrol.c             |   32 +++++++++++++++++++-------------
> >  mm/page_cgroup.c            |    2 +-
> >  3 files changed, 22 insertions(+), 15 deletions(-)
> > 
> > Index: mmotm-0727/include/linux/page_cgroup.h
> > ===================================================================
> > --- mmotm-0727.orig/include/linux/page_cgroup.h
> > +++ mmotm-0727/include/linux/page_cgroup.h
> > @@ -12,7 +12,8 @@
> >   */
> >  struct page_cgroup {
> >  	unsigned long flags;
> > -	struct mem_cgroup *mem_cgroup;
> > +	unsigned short mem_cgroup;	/* ID of assigned memory cgroup */
> > +	unsigned short blk_cgroup;	/* Not Used..but will be. */
> >  	struct page *page;
> >  	struct list_head lru;		/* per cgroup LRU list */
> >  };
> 
> Can I recommend that on 64 bit systems, we merge the flag, mem_cgroup
> and blk_cgroup into one 8 byte value. We could use
> __attribute("packed") and do something like this
> 

It's a next step.

> struct page_cgroup {
>         unsigned int flags;
>         unsigned short mem_cgroup;
>         unsigned short blk_cgroup;
>         ...
> } __attribute(("packed"));
> 
> Then we need to make sure we don't use more that 32 bits for flags,
> which is very much under control at the moment.
> 
set_bit() requires "long" as its argument. more some trick is required.

 And, IIUC, packing implies
	pc->mem_cgroup = mem_cgroup_id; or
	pc->blk_cgroup = blk_cgroup_id; will have race with
	set/clear_bit(BIT_XXXX, &pc->flags)
 
 This "packing" is not very easy. we have to consider all possible combinations
 of operations.

> This will save us 8 bytes in total on 64 bit systems and nothing on 32
> bit systems, but will enable blkio cgroup to co-exist.
> 

yes. But I have cocnerns of race condition. to do that, we need
patch 3-5. (But patch 5 adds spinlock, then no 8bytes reduce.)

Let me go step by step. I'm _really_ afraid of race conditions.

Thanks,
-Kame

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