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

[Andrea Righi]

On Wed, Mar 03, 2010 at 08:21:07AM +0900, Daisuke Nishimura wrote:
> On Tue, 2 Mar 2010 23:18:23 +0100, Andrea Righi <arighi at develer.com> wrote:
> > On Tue, Mar 02, 2010 at 07:20:26PM +0530, Balbir Singh wrote:
> > > * 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. 
> > 
> > I've started to look at dirty memory migration. First attempt is to add
> > DIRTY, WRITEBACK, etc. to page_cgroup flags and handle them in
> > __mem_cgroup_move_account(). Probably I'll have something ready for the
> > next version of the patch. I still need to figure if this can work as
> > expected...
> > 
> I agree it's a right direction(in fact, I have been planning to post a patch
> in that direction), so I leave it to you.
> Can you add PCG_FILE_MAPPED flag too ? I think this flag can be handled in the
> same way as other flags you're trying to add, and we can change
> "if (page_mapped(page) && !PageAnon(page))" to "if (PageCgroupFileMapped(pc)"
> in __mem_cgroup_move_account(). It would be cleaner than current code, IMHO.

OK, sounds good to me. I'll introduce PCG_FILE_MAPPED in the next
version.

Thanks,
-Andrea
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