linux-kernel - Re: [patch 0/2] mm: too_many_isolated can stall due to out of sync VM counters

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Date:   Tue, 14 Nov 2023 09:20:09 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Peter Xu <peterx@...hat.com>
Subject: Re: [patch 0/2] mm: too_many_isolated can stall due to out of sync
 VM counters

On Mon 13-11-23 20:34:20, Marcelo Tosatti wrote:
> A customer reported seeing processes hung at too_many_isolated,
> while analysis indicated that the problem occurred due to out
> of sync per-CPU stats (see below).
> 
> Fix is to use node_page_state_snapshot to avoid the out of stale values.
> 
> 2136 static unsigned long
>     2137 shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
>     2138                      struct scan_control *sc, enum lru_list lru)
>     2139 {
>     :
>     2145         bool file = is_file_lru(lru);
>     :
>     2147         struct pglist_data *pgdat = lruvec_pgdat(lruvec);
>     :
>     2150         while (unlikely(too_many_isolated(pgdat, file, sc))) {
>     2151                 if (stalled)
>     2152                         return 0;
>     2153
>     2154                 /* wait a bit for the reclaimer. */
>     2155                 msleep(100);   <--- some processes were sleeping here, with pending SIGKILL.
>     2156                 stalled = true;
>     2157
>     2158                 /* We are about to die and free our memory. Return now. */
>     2159                 if (fatal_signal_pending(current))
>     2160                         return SWAP_CLUSTER_MAX;
>     2161         }
> 
> msleep() must be called only when there are too many isolated pages:

What do you mean here?

>     2019 static int too_many_isolated(struct pglist_data *pgdat, int file,
>     2020                 struct scan_control *sc)
>     2021 {
>     :
>     2030         if (file) {
>     2031                 inactive = node_page_state(pgdat, NR_INACTIVE_FILE);
>     2032                 isolated = node_page_state(pgdat, NR_ISOLATED_FILE);
>     2033         } else {
>     :
>     2046         return isolated > inactive;
> 
> The return value was true since:
> 
>     crash> p ((struct pglist_data *) 0xffff00817fffe580)->vm_stat[NR_INACTIVE_FILE]
>     $8 = {
>       counter = 1
>     }
>     crash> p ((struct pglist_data *) 0xffff00817fffe580)->vm_stat[NR_ISOLATED_FILE]
>     $9 = {
>       counter = 2
> 
> while per_cpu stats had:
> 
>     crash> p ((struct pglist_data *) 0xffff00817fffe580)->per_cpu_nodestats
>     $85 = (struct per_cpu_nodestat *) 0xffff8000118832e0
>     crash> p/x 0xffff8000118832e0 + __per_cpu_offset[42]
>     $86 = 0xffff00917fcc32e0
>     crash> p ((struct per_cpu_nodestat *) 0xffff00917fcc32e0)->vm_node_stat_diff[NR_ISOLATED_FILE]
>     $87 = -1 '\377'
> 
>     crash> p/x 0xffff8000118832e0 + __per_cpu_offset[44]
>     $89 = 0xffff00917fe032e0
>     crash> p ((struct per_cpu_nodestat *) 0xffff00917fe032e0)->vm_node_stat_diff[NR_ISOLATED_FILE]
>     $91 = -1 '\377'

This doesn't really tell much. How much out of sync they really are
cumulatively over all cpus?
 
> It seems that processes were trapped in direct reclaim/compaction loop
> because these nodes had few free pages lower than watermark min.
> 
>   crash> kmem -z | grep -A 3 Normal
>   :
>   NODE: 4  ZONE: 1  ADDR: ffff00817fffec40  NAME: "Normal"
>     SIZE: 8454144  PRESENT: 98304  MIN/LOW/HIGH: 68/166/264
>     VM_STAT:
>           NR_FREE_PAGES: 68
>   --
>   NODE: 5  ZONE: 1  ADDR: ffff00897fffec40  NAME: "Normal"
>     SIZE: 118784  MIN/LOW/HIGH: 82/200/318
>     VM_STAT:
>           NR_FREE_PAGES: 45
>   --
>   NODE: 6  ZONE: 1  ADDR: ffff00917fffec40  NAME: "Normal"
>     SIZE: 118784  MIN/LOW/HIGH: 82/200/318
>     VM_STAT:
>           NR_FREE_PAGES: 53
>   --
>   NODE: 7  ZONE: 1  ADDR: ffff00997fbbec40  NAME: "Normal"
>     SIZE: 118784  MIN/LOW/HIGH: 82/200/318
>     VM_STAT:
>           NR_FREE_PAGES: 52

How have you concluded that too_many_isolated is at root of this issue.
With a very low NR_FREE_PAGES and many contending allocation the system
could be easily stuck in reclaim. What are other reclaim
characteristics? Is the direct reclaim successful? 

-- 
Michal Hocko
SUSE Labs