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    SubjectRe: [PATCH -mm 2/5] use ID in page cgroup
    On Tue, 3 Aug 2010 09:15:13 +0530
    Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

    > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2010-08-02 19:14:10]:
    >
    > > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    > >
    > > 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@jp.fujitsu.com>
    > > ---
    > > 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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