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

[Michal Hocko]

On Tue 15-09-20 08:48:08, Vijay Balakrishna wrote:
> 
> 
> 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 do not follow. If you are hotremoving during shutdown then how come
the value of min_free_kbytes matter at all?

> > 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!

Do you have the full oom splat? The fact that previous oom killer
invocations haven't helped and all the eligible tasks have been killed
and you still hit the oom would suggest there is a lot of memory
allocated without a direct relation to tasks. I fail to see how
min_free_kbytes would be related.

> 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.

The auto tuned value should never reach such a low value to cause
problems.

-- 
Michal Hocko
SUSE Labs