Messages in this thread | | | Date | Thu, 12 Mar 2009 15:15:29 +0530 | From | Balbir Singh <> | Subject | Re: [BUGFIX][PATCH 1/5] memcg use correct scan number at reclaim |
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* KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> [2009-03-12 16:45:59]:
> > On Thu, 12 Mar 2009 09:44:14 +0530 > > Balbir Singh <balbir@linux.vnet.ibm.com> wrote: > > > > > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-03-12 13:05:56]: > > > > > > > On Thu, 12 Mar 2009 09:30:54 +0530 > > > > Balbir Singh <balbir@linux.vnet.ibm.com> wrote: > > > > > > > > > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-03-12 12:51:24]: > > > > > > > > > > > On Thu, 12 Mar 2009 09:19:18 +0530 > > > > > > Balbir Singh <balbir@linux.vnet.ibm.com> wrote: > > > > > > > > > > > > > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-03-12 09:55:16]: > > > > > > > > > > > > > > > Andrew, this [1/5] is a bug fix, others are not. > > > > > > > > > > > > > > > > == > > > > > > > > From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> > > > > > > > > > > > > > > > > Even when page reclaim is under mem_cgroup, # of scan page is determined by > > > > > > > > status of global LRU. Fix that. > > > > > > > > > > > > > > > > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> > > > > > > > > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> > > > > > > > > --- > > > > > > > > mm/vmscan.c | 2 +- > > > > > > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > > > > > > > > > > > Index: mmotm-2.6.29-Mar10/mm/vmscan.c > > > > > > > > =================================================================== > > > > > > > > --- mmotm-2.6.29-Mar10.orig/mm/vmscan.c > > > > > > > > +++ mmotm-2.6.29-Mar10/mm/vmscan.c > > > > > > > > @@ -1470,7 +1470,7 @@ static void shrink_zone(int priority, st > > > > > > > > int file = is_file_lru(l); > > > > > > > > int scan; > > > > > > > > > > > > > > > > - scan = zone_page_state(zone, NR_LRU_BASE + l); > > > > > > > > + scan = zone_nr_pages(zone, sc, l); > > > > > > > > > > > > > > I have the exact same patch in my patch queue. BTW, mem_cgroup_zone_nr_pages is > > > > > > > buggy. We don't hold any sort of lock while extracting > > > > > > > MEM_CGROUP_ZSTAT (ideally we need zone->lru_lock). Without that how do > > > > > > > we guarantee that MEM_CGRUP_ZSTAT is not changing at the same time as > > > > > > > we are reading it? > > > > > > > > > > > > > Is it big problem ? We don't need very precise value and ZSTAT just have > > > > > > increment/decrement. So, I tend to ignore this small race. > > > > > > (and it's unsigned long, not long long.) > > > > > > > > > > > > > > > > The assumption is that unsigned long read is atomic even on 32 bit > > > > > systems? What if we get pre-empted in the middle of reading the data > > > > > and don't return back for long? The data can be highly in-accurate. > > > > > No? > > > > > > > > > Hmm, preempt_disable() is appropriate ? > > > > > > > > But shrink_zone() itself works on the value which is read at this time and > > > > dont' take care of changes in situation by preeemption...so it's not problem > > > > of memcg. > > > > > > > > > > You'll end up reclaiming based on old stale data. shrink_zone itself > > > maintains atomic data for zones. > > > > > IIUC, # of pages to be scanned is just determined once, here. > > In this case, lockless is right behavior. > lockless is valuable than precise ZSTAT. end user can't observe this race. >
Lockless works fine provided the data is correctly aligned. I need to check this out more thoroghly.
-- Balbir
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