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)? > 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. > >> 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. Even we can lost chance of clearing congestion flag of the zone. It would be a another patch. 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. -- Kind regards, Minchan Kim -- 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>