[Devel] Re: [PATCH -mmotm 3/3] memcg: dirty pages instrumentation

[KAMEZAWA Hiroyuki]

On Tue, 2 Mar 2010 09:01:58 +0100
Andrea Righi <arighi at develer.com> wrote:

> On Tue, Mar 02, 2010 at 09:23:09AM +0900, KAMEZAWA Hiroyuki wrote:
> > On Mon,  1 Mar 2010 22:23:40 +0100
> > Andrea Righi <arighi at develer.com> wrote:
> > 
> > > Apply the cgroup dirty pages accounting and limiting infrastructure to
> > > the opportune kernel functions.
> > > 
> > > Signed-off-by: Andrea Righi <arighi at develer.com>
> > 
> > Seems nice.
> > 
> > Hmm. the last problem is moving account between memcg.
> > 
> > Right ?
> 
> Correct. This was actually the last item of the TODO list. Anyway, I'm
> still considering if it's correct to move dirty pages when a task is
> migrated from a cgroup to another. Currently, dirty pages just remain in
> the original cgroup and are flushed depending on the original cgroup
> settings. That is not totally wrong... at least moving the dirty pages
> between memcgs should be optional (move_charge_at_immigrate?).
> 

My concern is 
 - migration between memcg is already suppoted
    - at task move
    - at rmdir

Then, if you leave DIRTY_PAGE accounting to original cgroup,
the new cgroup (migration target)'s Dirty page accounting may
goes to be negative, or incorrect value. Please check FILE_MAPPED
implementation in __mem_cgroup_move_account()

As
       if (page_mapped(page) && !PageAnon(page)) {
                /* Update mapped_file data for mem_cgroup */
                preempt_disable();
                __this_cpu_dec(from->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
                __this_cpu_inc(to->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
                preempt_enable();
        }
then, FILE_MAPPED never goes negative.


Thanks,
-Kame

> Thanks for your ack and the detailed review!
> 
> -Andrea
> 
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