Re: [RFC PATCH v7 2/2] mm: Fix OOM killer inaccuracy on large many-core systems

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2025-11-06 19:32, Shakeel Butt wrote:

[...]

>> [   14.858862][   T67] BUG: Bad rss-counter state mm:ffff8881000655c0 type:MM_ANONPAGES val:0 Comm:kworker/u9:0 Pid:67
>> [   14.894890][   T69] BUG: Bad rss-counter state mm:ffff888100061cc0 type:MM_FILEPAGES val:0 Comm:kworker/u9:0 Pid:69
>> [   14.896108][   T69] BUG: Bad rss-counter state mm:ffff888100061cc0 type:MM_ANONPAGES val:0 Comm:kworker/u9:0 Pid:69
> 
> Hmm this shows that percpu_counter_tree_precise_sum() is returning 0 but
> percpu_counter_tree_approximate_sum() is off more than
> counter->inaccuracy. I have not dig deeper to find why but this needs to
> be resolved before considering this series for upstream.

I notice that those BUG show up while loading modules at boot in kworker context, e.g.:

[   14.858862][   T67] BUG: Bad rss-counter state mm:ffff8881000655c0 type:MM_ANONPAGES val:0 Comm:kworker/u9:0 Pid:67
[   14.894890][   T69] BUG: Bad rss-counter state mm:ffff888100061cc0 type:MM_FILEPAGES val:0 Comm:kworker/u9:0 Pid:69
[   14.896108][   T69] BUG: Bad rss-counter state mm:ffff888100061cc0 type:MM_ANONPAGES val:0 Comm:kworker/u9:0 Pid:69
[   14.918858][   T71] module: module-autoload: duplicate request for module crypto-aes
[   14.919479][   T71] module: module-autoload: duplicate request for module crypto-aes-all
[   14.920801][    T1] krb5: Running aes128-cts-hmac-sha256-128 enc plain<block
[   14.921844][    T1] krb5: Running aes128-cts-hmac-sha256-128 enc plain==block
[   14.922852][    T1] krb5: Running aes128-cts-hmac-sha256-128 enc plain>block
[   14.923843][    T1] krb5: Running aes256-cts-hmac-sha384-192 enc no plain
[   14.939591][    T1] krb5: Running aes256-cts-hmac-sha384-192 enc plain<block
[   14.940614][    T1] krb5: Running aes256-cts-hmac-sha384-192 enc plain==block
[   14.941586][    T1] krb5: Running aes256-cts-hmac-sha384-192 enc plain>block
[   14.942547][    T1] krb5: Running camellia128-cts-cmac enc no plain
[   15.018568][   T85] BUG: Bad rss-counter state mm:ffff888160f81340 type:MM_ANONPAGES val:0 Comm:kworker/u9:0 Pid:85b

I used "module_init" similarly to lib/percpu_counter.c, but I think it
happens too late in the boot sequence:

   module_init(percpu_counter_startup);

module_init maps to __initcall within a built-in compile unit, which
maps to device_initcall(), which happens quite late within the sequence
called from do_initcalls(), called from do_basic_setup().

And even do_basic_setup is documented as:

  * Ok, the machine is now initialized. None of the devices
  * have been touched yet, but the CPU subsystem is up and
  * running, and memory and process management works.

which clearly requires that the mm subsystem is expected to
be ready at that point.

It probably was not an issue for the non-hierarchical percpu
counters because all it was initializing is handling of CPU hotplug,
but the new hierarchical counters initialize the pre-calculated
inaccuracy value which is used to figure out whether the approximate
sum is sufficient to compare values or if the precise sum is needed.

I think this is why we are hitting this BUG.

Now I wonder where I should move this initialization. It requires
"nr_cpu_ids" to be initialized, and pretty much need to be done
before mms are created. I'm starting to suspect that the module init
code can spawn kworkers that have a mm before the init process runs.

Thoughts ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com