Messages in this thread | | | Subject | Re: [PATCH v2] mm/vmscan: fix high cpu usage of kswapd if there are no reclaimable pages | From | hejianet <> | Date | Tue, 28 Feb 2017 09:53:20 +0800 |
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Hi Johannes
I have another concern: kswapd -> balance_pgdat -> age_active_anon This code path will do some background works to age anon list, will this patch have some impact on it if the retry time is > 16 and kswapd is not waken up?
B.R. Jia
On 28/02/2017 1:06 AM, Johannes Weiner wrote: > On Mon, Feb 27, 2017 at 09:50:24AM +0100, Michal Hocko wrote: >> On Fri 24-02-17 11:51:05, Johannes Weiner wrote: >> [...] >>> >From 29fefdca148e28830e0934d4e6cceb95ed2ee36e Mon Sep 17 00:00:00 2001 >>> From: Johannes Weiner <hannes@cmpxchg.org> >>> Date: Fri, 24 Feb 2017 10:56:32 -0500 >>> Subject: [PATCH] mm: vmscan: disable kswapd on unreclaimable nodes >>> >>> Jia He reports a problem with kswapd spinning at 100% CPU when >>> requesting more hugepages than memory available in the system: >>> >>> $ echo 4000 >/proc/sys/vm/nr_hugepages >>> >>> top - 13:42:59 up 3:37, 1 user, load average: 1.09, 1.03, 1.01 >>> Tasks: 1 total, 1 running, 0 sleeping, 0 stopped, 0 zombie >>> %Cpu(s): 0.0 us, 12.5 sy, 0.0 ni, 85.5 id, 2.0 wa, 0.0 hi, 0.0 si, 0.0 st >>> KiB Mem: 31371520 total, 30915136 used, 456384 free, 320 buffers >>> KiB Swap: 6284224 total, 115712 used, 6168512 free. 48192 cached Mem >>> >>> PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND >>> 76 root 20 0 0 0 0 R 100.0 0.000 217:17.29 kswapd3 >>> >>> At that time, there are no reclaimable pages left in the node, but as >>> kswapd fails to restore the high watermarks it refuses to go to sleep. >>> >>> Kswapd needs to back away from nodes that fail to balance. Up until >>> 1d82de618ddd ("mm, vmscan: make kswapd reclaim in terms of nodes") >>> kswapd had such a mechanism. It considered zones whose theoretically >>> reclaimable pages it had reclaimed six times over as unreclaimable and >>> backed away from them. This guard was erroneously removed as the patch >>> changed the definition of a balanced node. >>> >>> However, simply restoring this code wouldn't help in the case reported >>> here: there *are* no reclaimable pages that could be scanned until the >>> threshold is met. Kswapd would stay awake anyway. >>> >>> Introduce a new and much simpler way of backing off. If kswapd runs >>> through MAX_RECLAIM_RETRIES (16) cycles without reclaiming a single >>> page, make it back off from the node. This is the same number of shots >>> direct reclaim takes before declaring OOM. Kswapd will go to sleep on >>> that node until a direct reclaimer manages to reclaim some pages, thus >>> proving the node reclaimable again. >> >> Yes this looks, nice&simple. I would just be worried about [1] a bit. >> Maybe that is worth a separate patch though. >> >> [1] http://lkml.kernel.org/r/20170223111609.hlncnvokhq3quxwz@dhcp22.suse.cz > > I think I'd prefer the simplicity of keeping this contained inside > vmscan.c, as an interaction between direct reclaimers and kswapd, as > well as leaving the wakeup tied to actually seeing reclaimable pages > rather than merely producing free pages (e.g. should we also add a > kick to a large munmap() for example?). > > OOM kills come with such high latencies that I cannot imagine a > slightly quicker kswapd restart would matter in practice. > >>> Reported-by: Jia He <hejianet@gmail.com> >>> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org> >> >> Acked-by: Michal Hocko <mhocko@suse.com> > > Thanks! > >> I would have just one more suggestion. Please move MAX_RECLAIM_RETRIES >> to mm/internal.h. This is MM internal thing and there is no need to make >> it visible. > > Good point, I'll move it. >
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