On Thu, 18 Mar 2010 08:54:11 +0900 KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote: > On Wed, 17 Mar 2010 17:28:55 +0530 > Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote: > > > * Andrea Righi <arighi@xxxxxxxxxxx> [2010-03-15 00:26:38]: > > > > > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> > > > > > > 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. 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>