On Wed, Jul 20, 2011 at 2:18 PM, Shaohua Li <shaohua.li@xxxxxxxxx> wrote: > 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. In case of higher order zone reclaiming, we just make sure 25% of zones are balanced but not for order-0 on *all* zones. > >> 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. We do in not-high-order allocation as well as high order allocation if the zone is watermark_okay. > >> 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(). Okay. I will put it in my TODO. Thanks, Shaohua. -- 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>