Re: [PATCH bpf-next v3 01/13] bpf: Add support for non-fix-size percpu mem allocation

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On 11/15/23 8:15 PM, Hou Tao wrote:

On 11/15/2023 11:31 PM, Heiko Carstens wrote:
On Sun, Aug 27, 2023 at 08:27:34AM -0700, Yonghong Song wrote:
This is needed for later percpu mem allocation when the
allocation is done by bpf program. For such cases, a global
bpf_global_percpu_ma is added where a flexible allocation
size is needed.

Signed-off-by: Yonghong Song <>
  include/linux/bpf.h   |  4 ++--
  kernel/bpf/core.c     |  8 +++++---
  kernel/bpf/memalloc.c | 14 ++++++--------
  3 files changed, 13 insertions(+), 13 deletions(-)
Both Marc and Mikhail reported out-of-memory conditions on s390 machines,
and bisected it down to this upstream commit 41a5db8d8161 ("bpf: Add
support for non-fix-size percpu mem allocation").
This seems to eat up a lot of memory only based on the number of possible

If we have a machine with 8GB, 6 present CPUs and 512 possible CPUs (yes,
this is a realistic scenario) the memory consumption directly after boot

$ cat /sys/devices/system/cpu/present
$ cat /sys/devices/system/cpu/possible
Will the present CPUs be hot-added dynamically and eventually increase
to 512 CPUs ? Or will the present CPUs rarely be hot-added ? After all
possible CPUs are online, will these CPUs be hot-plugged dynamically ?
Because I am considering add CPU hotplug support for bpf mem allocator,
so we can allocate memory according to the present CPUs instead of
possible CPUs. But if the present CPUs will be increased to all possible
CPUs quickly, there will be not too much benefit to support hotplug in
bpf mem allocator.

Thanks, Hou. In my development machine and also checking
some internal production machines, no suprisingly, the 'present' number
cpus is equal to the 'possible' number of cpus. I suspect in most cases,
'present' and 'possible' are the same. But it would be good that
other people can share their 'present != possible' configuration
and explain why.

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