Re: [[PATCH]] mm: khugepaged: recalculate min_free_kbytes after memory hotplug as expected by khugepaged

[Vijay Balakrishna <vijayb@linux.microsoft.com>]

On 9/15/2020 1:18 AM, Michal Hocko wrote:
> On Mon 14-09-20 09:57:02, Vijay Balakrishna wrote:
>>
>>
>> On 9/14/2020 7:33 AM, Michal Hocko wrote:
>>> On Thu 10-09-20 13:47:39, Vijay Balakrishna wrote:
>>>> When memory is hotplug added or removed the min_free_kbytes must be
>>>> recalculated based on what is expected by khugepaged.  Currently
>>>> after hotplug, min_free_kbytes will be set to a lower default and higher
>>>> default set when THP enabled is lost. This leaves the system with small
>>>> min_free_kbytes which isn't suitable for systems especially with network
>>>> intensive loads.  Typical failure symptoms include HW WATCHDOG reset,
>>>> soft lockup hang notices, NETDEVICE WATCHDOG timeouts, and OOM process
>>>> kills.
>>>
>>> Care to explain some more please? The whole point of increasing
>>> min_free_kbytes for THP is to get a larger free memory with a hope that
>>> huge pages will be more likely to appear. While this might help for
>>> other users that need a high order pages it is definitely not the
>>> primary reason behind it. Could you provide an example with some more
>>> data?
>>
>> Thanks Michal.  I haven't looked into THP as part of my investigation, so I
>> cannot comment.
>>
>> In our use case we are hotplug removing ~2GB of 8GB total (on our SoC)
>> during normal reboot/shutdown.  This memory is hotplug hot-added as movable
>> type via systemd late service during start-of-day.
>>
>> In our stress test first we ran into HW WATCHDOG recovery, on enabling
>> kernel watchdog we started seeing soft lockup hung task notices, failure
>> symptons varied, where stack trace of hung tasks sometimes trying to
>> allocate GFP_ATOMIC memory, looping in do_notify_resume, NETDEVICE WATCHDOG
>> timeouts, OOM process kills etc.,  During investigation we reran stress test
>> without hotplug use case.  Surprisingly this run didn't encounter the said
>> problems.  This led to comparing what is different between the two runs,
>> while looking at various globals, studying hotplug code I uncovered the
>> issue of failing to restore min_free_kbytes.  In particular on our 8GB SoC
>> min_free_kbytes went down to 8703 from 22528 after hotplug add.
> 
> Did you try to increase min_free_kbytes manually after hot remove? Btw.

No, in our use case memory hot remove done during shutdown.

> I would consider oom killer invocation due to min_free_kbytes really
> weird behavior. If anything the higher value would cause more memory
> reclaim and potentially oom rather than smaller one.

Yes, we wondered about it too.  One panic stack trace (after many OOM kills)

[330321.174240] Out of memory and no killable processes...
[330321.179658] Kernel panic - not syncing: System is deadlocked on memory
[330321.186489] CPU: 4 PID: 1 Comm: systemd Kdump: loaded Tainted: G 
       O      5.4.51-xxx #1
[330321.196900] Hardware name: Overlake (DT)
[330321.201038] Call trace:
[330321.203660]  dump_backtrace+0x0/0x1d0
[330321.207533]  show_stack+0x20/0x2c
[330321.211048]  dump_stack+0xe8/0x150
[330321.214656]  panic+0x18c/0x3b4
[330321.217901]  out_of_memory+0x4c0/0x6e4
[330321.221863]  __alloc_pages_nodemask+0xbdc/0x1c90
[330321.226722]  alloc_pages_current+0x21c/0x2b0
[330321.231220]  alloc_slab_page+0x1e0/0x7d8
[330321.235361]  new_slab+0x2e8/0x2f8
[330321.238874]  ___slab_alloc+0x45c/0x59c
[330321.242835]  kmem_cache_alloc+0x2d4/0x360
[330321.247065]  getname_flags+0x6c/0x2a8
[330321.250938]  user_path_at_empty+0x3c/0x68
[330321.255168]  do_readlinkat+0x7c/0x17c
[330321.259039]  __arm64_sys_readlinkat+0x5c/0x70
[330321.263627]  el0_svc_handler+0x1b8/0x32c
[330321.267767]  el0_svc+0x10/0x14
[330321.271026] SMP: stopping secondary CPUs
[330321.275382] Starting crashdump kernel...
[330321.279526] Bye!

Then while searching I came across documented warning below.  In above 
instance panic after OOM kills happened after 3+ days of stress run (a 
mixure of ttcp, cpuloadgen and fio).

https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/performance_tuning_guide/sect-red_hat_enterprise_linux-performance_tuning_guide-configuration_tools-configuring_system_memory_capacity

Warning

Extreme values can damage your system. Setting min_free_kbytes to an 
extremely low value prevents the system from reclaiming memory, which 
can result in system hangs and OOM-killing processes. However, setting 
min_free_kbytes too high (for example, to 5–10% of total system memory) 
causes the system to enter an out-of-memory state immediately, resulting 
in the system spending too much time reclaiming memory.