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Date: Mon, 23 May 2016 09:29:04 +0200 From: Michal Hocko <mhocko@...nel.org> To: Oleg Nesterov <oleg@...hat.com> Cc: Andrew Morton <akpm@...ux-foundation.org>, Andrea Arcangeli <aarcange@...hat.com>, Mel Gorman <mgorman@...hsingularity.net>, linux-kernel@...r.kernel.org, linux-mm@...ck.org Subject: Re: zone_reclaimable() leads to livelock in __alloc_pages_slowpath() Hi, Tetsuo has already pointed you at my oom detection rework which removes the zone_reclaimable ugliness (btw. one of the top reasons to rework this area) and it is likely to fix your problem. I would still like to understand what happens with your test case because we might want to prepare a stable patch for older kernels. On Fri 20-05-16 22:28:17, Oleg Nesterov wrote: > I don't understand vmscan.c, and in fact I don't even understand NR_PAGES_SCANNED [...] > counter... why it has to be atomic/per-cpu? It is always updated under ->lru_lock > except free_pcppages_bulk/free_one_page try to reset this counter. But note that > they both do It doesn't really have to be atomic/per-cpu because it is really updated under the lock. It just uses the generic vmstat infrastructure... > nr_scanned = zone_page_state(zone, NR_PAGES_SCANNED); > if (nr_scanned) > __mod_zone_page_state(zone, NR_PAGES_SCANNED, -nr_scanned); > > and this doesn't look exactly right: zone_page_state() ignores the per-cpu > ->vm_stat_diff[] counters (and we probably do not want for_each_online_cpu() > loop here). And I do not know if this is really bad or not, but note that if > I change calculate_normal_threshold() to return 0, the problem goes away too. You are absolutely right that this is racy. In the worst case we would end up missing nr_cpus*threshold scanned pages which would stay behind. But bool zone_reclaimable(struct zone *zone) { return zone_page_state_snapshot(zone, NR_PAGES_SCANNED) < zone_reclaimable_pages(zone) * 6; } So the left over shouldn't cause it to return true all the time. In fact it could prematurely say false, right? (note that _snapshot variant considers per-cpu diffs [1]). That being said I am not really sure why would the 0 threshold help for your test case. Could you add some tracing and see what are the numbers above? Is it possible that zone_reclaimable_pages is some small number which actuall prevents us to scan anything? Aka a bug is get_scan_count or somewhere else? [1] I am not really sure which kernel version have you tested - your config says 4.6.0-rc7 but this is true since 0db2cb8da89d ("mm, vmscan: make zone_reclaimable_pages more precise") which is 4.6-rc1. -- Michal Hocko SUSE Labs
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