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

Marcelo Tosatti
2023-11-14T12:27:34Z

Hi Michal,

On Tue, Nov 14, 2023 at 09:20:09AM +0100, Michal Hocko wrote:
> 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?

That msleep() must not be called when

isolated > inactive

is false.

> > 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?

This is the cumulative value over all CPUs (offsets for other CPUs
have been omitted since they are zero).

> > 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.

Because the customer observed the problem and obtained traces:

"If so, I have to mention about an another problem caused by vmstat issue here.
The customer experienced process hang like the issue reported here, but in this case the
process was trapped in compaction route. In shrink_inactive_list(), reclaim_throttle() is called
when too_many_isolated() is true. In fact confirmed from memory dump, there was no isolated
pages but zone's vmstat have 2 counts as isolated pages and percpu vmstats have -2 counts.

too_many = isolated > (inactive + active) / 2;

There was no more inactive and active pages. As the result, the process was throttled in
this point again and again until finish of parallel reclaimers who did not exist there in real."

> With a very low NR_FREE_PAGES and many contending allocation the system
> could be easily stuck in reclaim. What are other reclaim
> characteristics?

I can ask. What information in particular do you want to know?

> Is the direct reclaim successful?

Processes are stuck in too_many_isolated (unnecessarily). What do you mean when you ask
"Is the direct reclaim successful", precisely?