* nishim...@mxp.nes.nec.co.jp <nishim...@mxp.nes.nec.co.jp> [2010-03-18
11:16:53]:
> On Thu, 18 Mar 2010 09:45:19 +0900, KAMEZAWA Hiroyuki
> <kamezawa.hir...@jp.fujitsu.com> wrote:
> > On Thu, 18 Mar 2010 08:54:11 +0900
> > KAMEZAWA Hiroyuki <kamezawa.hir...@jp.fujitsu.com> wrote:
> >
> > > On Wed, 17 Mar 2010 17:28:55 +0530
> > > Balbir Singh <bal...@linux.vnet.ibm.com> wrote:
> > >
> > > > * Andrea Righi <ari...@develer.com> [2010-03-15 00:26:38]:
> > > >
> > > > > From: KAMEZAWA Hiroyuki <kamezawa.hir...@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.
>
True
>
> > 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.
Agreed, we need to find a simpler way of doing this without affecting
the accuracy of accounting - may be two accounting routines for two
code paths. I have not thought through this yet.
--
Three Cheers,
Balbir
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