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SubjectMachine lockups on extreme memory pressure
Hi all,

We are seeing machine lockups due extreme memory pressure where the
free pages on all the zones are way below the min watermarks. The stack
of the stuck CPU looks like the following (I had to crash the machine to
get the info).

#0 [ ] crash_nmi_callback
#1 [ ] nmi_handle
#2 [ ] default_do_nmi
#3 [ ] do_nmi
#4 [ ] end_repeat_nmi
--- <NMI exception stack> ---
#5 [ ] queued_spin_lock_slowpath
#6 [ ] _raw_spin_lock
#7 [ ] ____cache_alloc_node
#8 [ ] fallback_alloc
#9 [ ] __kmalloc_node_track_caller
#10 [ ] __alloc_skb
#11 [ ] tcp_send_ack
#12 [ ] tcp_delack_timer
#13 [ ] run_timer_softirq
#14 [ ] irq_exit
#15 [ ] smp_apic_timer_interrupt
#16 [ ] apic_timer_interrupt
--- <IRQ stack> ---
#17 [ ] apic_timer_interrupt
#18 [ ] _raw_spin_lock
#19 [ ] vmpressure
#20 [ ] shrink_node
#21 [ ] do_try_to_free_pages
#22 [ ] try_to_free_pages
#23 [ ] __alloc_pages_direct_reclaim
#24 [ ] __alloc_pages_nodemask
#25 [ ] cache_grow_begin
#26 [ ] fallback_alloc
#27 [ ] __kmalloc_node_track_caller
#28 [ ] __alloc_skb
#29 [ ] tcp_sendmsg_locked
#30 [ ] tcp_sendmsg
#31 [ ] inet6_sendmsg
#32 [ ] ___sys_sendmsg
#33 [ ] sys_sendmsg
#34 [ ] do_syscall_64

These are high traffic machines. Almost all the CPUs are stuck on the
root memcg's vmpressure sr_lock and almost half of the CPUs are stuck
on kmem cache node's list_lock in the IRQ. Note that the vmpressure
sr_lock is irq-unsafe. Couple of months back, we observed a similar
situation with swap locks which forces us to disable swap on global
pressure. Since we do proactive reclaim disabling swap on global reclaim
was not an issue. However now we have started seeing the same situation
with other irq-unsafe locks like vmpressure sr_lock and almost all the
slab shrinkers have irq-unsafe spinlocks. One of way to mitigate this
is by converting all such locks (which can be taken in reclaim path)
to be irq-safe but it does not seem like a maintainable solution.

Please note that we are running user space oom-killer which is more
aggressive than oomd/PSI but even that got stuck under this much memory
pressure.

I am wondering if anyone else has seen a similar situation in production
and if there is a recommended way to resolve this situation.

thanks,
Shakeel

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