On Wed, 2011-07-20 at 12:09 +0800, Minchan Kim wrote: > On Wed, Jul 20, 2011 at 9:43 AM, Shaohua Li <shaohua.li@xxxxxxxxx> wrote: > > On Wed, 2011-07-20 at 00:51 +0800, Minchan Kim wrote: > >> On Tue, Jul 19, 2011 at 04:53:04PM +0800, Shaohua Li wrote: > >> > On Tue, 2011-07-19 at 16:45 +0800, Minchan Kim wrote: > >> > > On Tue, Jul 19, 2011 at 4:09 PM, Shaohua Li <shaohua.li@xxxxxxxxx> wrote: > >> > > > I'm running a workload which triggers a lot of swap in a machine with 4 nodes. > >> > > > After I kill the workload, I found a kswapd livelock. Sometimes kswapd3 or > >> > > > kswapd2 are keeping running and I can't access filesystem, but most memory is > >> > > > free. This looks like a regression since commit 08951e545918c159. > >> > > > >> > > Could you tell me what is 08951e545918c159? > >> > > You mean [ebd64e21ec5a, > >> > > mm-vmscan-only-read-new_classzone_idx-from-pgdat-when-reclaiming-successfully] > >> > > ? > >> > ha, sorry, I should copy the commit title. > >> > 08951e545918c159(mm: vmscan: correct check for kswapd sleeping in > >> > sleeping_prematurely) > >> > > >> > >> I don't mean it. In my bogus git tree, I can't find it but I can look at it in repaired git tree. :) > >> Anyway, I have a comment. Please look at below. > >> > >> On Tue, Jul 19, 2011 at 03:09:27PM +0800, Shaohua Li wrote: > >> > I'm running a workload which triggers a lot of swap in a machine with 4 nodes. > >> > After I kill the workload, I found a kswapd livelock. Sometimes kswapd3 or > >> > kswapd2 are keeping running and I can't access filesystem, but most memory is > >> > free. This looks like a regression since commit 08951e545918c159. > >> > Node 2 and 3 have only ZONE_NORMAL, but balance_pgdat() will return 0 for > >> > classzone_idx. The reason is end_zone in balance_pgdat() is 0 by default, if > >> > all zones have watermark ok, end_zone will keep 0. > >> > Later sleeping_prematurely() always returns true. Because this is an order 3 > >> > wakeup, and if classzone_idx is 0, both balanced_pages and present_pages > >> > in pgdat_balanced() are 0. > >> > >> Sigh. Yes. > >> > >> > We add a special case here. If a zone has no page, we think it's balanced. This > >> > fixes the livelock. > >> > >> Yes. Your patch can fix it but I don't like that it adds handling special case. > >> (Although Andrew merged quickly). > > The special case is reasonable, because if a zone has no page, it should > > be considered balanced. > > Yes. It's not bad and even simple but my concern is that at the moment > kswapd code is very complicated and it's not hot path so I would like > to add more readable code. > > > > >> The problem is to return 0-classzone_idx if all zones was okay. > >> So how about this? > > My original implementation is like this (I return a populated zone with > > minimum zone index). But I changed my mind later. the end_zone is zone > > we work, so return 0 is reasonable, because all zones are ok. Maybe we > > If it is reasonable, did you work on ZONE_DMA(zone index: 0)? return -1 can help. > > should return -1 if all zones are ok, but this is another story. > > I think that return classzone_id(-1) and handle such case is more readable. sure, we need another patch to clean up it. > > > >> This can change old behavior slightly. > >> For example, if balance_pgdat calls with order-3 and all zones are okay about order-3, > >> it will recheck order-0 as end_zone isn't 0 any more. > >> But I think it's desriable side effect we have missed. > > if order-3 is ok, order-0 is ok too I think, so the check is > > unnecessary. > > No. It's not for the zone but *zones. > In case of reclaiming higher order zone, it can sleep without all > zones being balanced so that precious order-0 of some zone would be > not balanced. when balance_pgdat() skips the loop for higher order zone, it already sets end_zone, so I thought this isn't a problem. > Even we can lost chance of clearing congestion flag of the zone. > It would be a another patch. yep, the congestion flag clearing is a bit confusing. I don't even know why just do it in high order allocation. If all zones are ok, we should clear the flag regardless the order. > In conclusion, I would like to avoid complicated thing but I am going > to be not against you strongly if other doesn't agree on me. > I might need a time to clean kswapd's spagetti up. ok, understand it. I have similar concerns actually. I thought my patch is simple enough to solve the livelock. But we do have space to cleanup balance_pgdat(). Thanks, Shaohua -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>