[Devel] Re: [PATCH -mmotm 1/5] memcg: disable irq at page cgroup lock

[Daisuke Nishimura]

On Thu, 18 Mar 2010 09:45:19 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com> wrote:
> On Thu, 18 Mar 2010 08:54:11 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com> wrote:
> 
> > On Wed, 17 Mar 2010 17:28:55 +0530
> > Balbir Singh <balbir at linux.vnet.ibm.com> wrote:
> > 
> > > * Andrea Righi <arighi at develer.com> [2010-03-15 00:26:38]:
> > > 
> > > > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
> > > > 
> > > > Now, file-mapped is maintaiend. But more generic update function
> > > > will be needed for dirty page accounting.
> > > > 
> > > > For accountig page status, we have to guarantee lock_page_cgroup()
> > > > will be never called under tree_lock held.
> > > > To guarantee that, we use trylock at updating status.
> > > > By this, we do fuzzy accounting, but in almost all case, it's correct.
> > > >
> > > 
> > > I don't like this at all, but in almost all cases is not acceptable
> > > for statistics, since decisions will be made on them and having them
> > > incorrect is really bad. Could we do a form of deferred statistics and
> > > fix this.
> > > 
> > 
> > plz show your implementation which has no performance regresssion.
> > For me, I don't neee file_mapped accounting, at all. If we can remove that,
> > we can add simple migration lock.
> > file_mapped is a feattue you added. please improve it.
> > 
> 
> BTW, I should explain how acculate this accounting is in this patch itself.
> 
> Now, lock_page_cgroup/unlock_page_cgroup happens when
> 	- charge/uncharge/migrate/move accounting
> 
> Then, the lock contention (trylock failure) seems to occur in conflict
> with
> 	- charge, uncharge, migarate. move accounting
> 
> About dirty accounting, charge/uncharge/migarate are operation in synchronous
> manner with radix-tree (holding treelock etc). Then no account leak.
> move accounting is only source for inacculacy...but I don't think this move-task
> is ciritial....moreover, we don't move any file pages at task-move, now.
> (But Nishimura-san has a plan to do so.)
> So, contention will happen only at confliction with force_empty.
> 
> About FILE_MAPPED accounting, it's not synchronous with radix-tree operaton.
> Then, accounting-miss seems to happen when charge/uncharge/migrate/account move.
> But
> 	charge .... we don't account a page as FILE_MAPPED before it's charged.
> 	uncharge .. usual operation in turncation is unmap->remove-from-radix-tree.
> 		    Then, it's sequential in almost all case. The race exists when...
> 		    Assume there are 2 threads A and B. A truncate a file, B map/unmap that.
> 		    This is very unusal confliction.
> 	migrate.... we do try_to_unmap before migrating pages. Then, FILE_MAPPED
> 		    is properly handled.
> 	move account .... we don't have move-account-mapped-file, yet.
> 
FILE_MAPPED is updated under pte lock. OTOH, move account is also done under
pte lock. page cgroup lock is held under pte lock in both cases, so move account
is not so problem as for FILE_MAPPED.


Thanks,
Daisuke Nishimura.

> Then, this trylock contention happens at contention with force_empty and truncate.
> 
> 
> Then, main issue for contention is force_empty. But it's called for removing memcg,
> accounting for such memcg is not important.
> Then, I say "this accounting is Okay."
> 
> To do more accurate, we may need another "migration lock". But to get better
> performance for root cgroup, we have to call mem_cgroup_is_root() before
> taking lock and there will be another complicated race.
> 
> Bye,
> -Kame
> 
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