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

[Yonghong Song <yonghong.song@linux.dev>]

On 11/15/23 8:15 PM, Hou Tao wrote:
> Hi,
>
> 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 <yonghong.song@linux.dev>
>>> ---
>>>   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
>> CPUs.
>>
>> 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
>> is:
>>
>> $ cat /sys/devices/system/cpu/present
>> 0-5
>> $ cat /sys/devices/system/cpu/possible
>> 0-511
> 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.