linux-kernel - Re: oom-killer not invoked on systems with multiple memory-tiers

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Message-ID: <CAC5umyiA+KkQNTv-mN=mA9BXHmqMH9Trr_07XYOh=QTLLm=zNg@mail.gmail.com>
Date: Thu, 30 Oct 2025 09:51:46 +0900
From: Akinobu Mita <akinobu.mita@...il.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: linux-kernel@...r.kernel.org, linux-cxl@...r.kernel.org, 
	linux-mm@...ck.org
Subject: Re: oom-killer not invoked on systems with multiple memory-tiers

2025年10月29日(水) 4:54 Shakeel Butt <shakeel.butt@...ux.dev>:
>
> Hi Akinobu,
>
> On Wed, Oct 22, 2025 at 10:57:35PM +0900, Akinobu Mita wrote:
> > On systems with multiple memory-tiers consisting of DRAM and CXL memory,
> > the OOM killer is not invoked properly.
> >
> > Here's the command to reproduce:
> >
> > $ stress-ng --oomable -v --memrate 20 --memrate-bytes 10G \
> >     --memrate-rd-mbs 1 --memrate-wr-mbs 1
> >
> > The memory usage is the number of workers specified with the --memrate
> > option multiplied by the buffer size specified with the --memrate-bytes
> > option, so please adjust it so that it exceeds the total size of the
> > installed DRAM and CXL memory.
> >
> > If swap is disabled, you can usually expect the OOM killer to terminate
> > the stress-ng process when memory usage approaches the installed memory size.
> >
> > However, if multiple memory-tiers exist (multiple
> > /sys/devices/virtual/memory_tiering/memory_tier<N> directories exist),
> > and /sys/kernel/mm/numa/demotion_enabled is true and
> > /sys/kernel/mm/lru_gen/min_ttl_ms is 0, the OOM killer will not be invoked
> > and the system will become inoperable.
> >
> > If /sys/kernel/mm/numa/demotion_enabled is false, or if demotion_enabled
> > is true but /sys/kernel/mm/lru_gen/min_ttl_ms is set to a non-zero value
> > such as 1000, the OOM killer will be invoked properly.
> >
> > This issue can be reproduced using NUMA emulation even on systems with
> > only DRAM. However, to configure multiple memory-tiers using fake nodes,
> > you must apply the attached patch.
> >
> > You can create two-fake memory-tiers by booting a single-node system with
> > the following boot options:
> >
> > numa=fake=2
> > numa_emulation.default_dram=1,0
> > numa_emulation.read_latency=100,1000
> > numa_emulation.write_latency=100,1000
> > numa_emulation.read_bandwidth=100000,10000
> > numa_emulation.write_bandwidth=100000,10000
> >
>
> Thanks for the report. Can you try to repro this with traditional LRU
> i.e. not MGLRU? I just want to see if this is MGLRU only issue or more
> general.

The problem was also reproduced with traditional LRU.
Here is a summary of the conditions for reproduction:

* Reproducible (The system becomes inoperable)
  * demotion_enabled=true lru_gen/enabled=0x0007,min_ttl_ms=0
  * demotion_enabled=true lru_gen/enabled=0x0000
* Unreproducible (OOM killer is invoked)
  * demotion_enabled=false
  * demotion_enabled=true lru_gen/enabled=0x0007,min_ttl_ms=1