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

[Balbir Singh]

* KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com> [2010-03-02 17:23:16]:

> 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.
>

Absolutely! I am not sure how complex dirty memory migration will be,
but one way of working around it would be to disable migration of
charges when the feature is enabled (dirty* is set in the memory
cgroup). We might need additional logic to allow that to happen. 

-- 
	Three Cheers,
	Balbir
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