Re: Do we still need SLAB_MEM_SPREAD (and possibly others)?

[Waiman Long <longman@redhat.com>]

On 2/5/24 22:16, Waiman Long wrote:
>
> On 2/5/24 20:46, Song, Xiongwei wrote:
>> Adding the maintainers of cpuset of cgroup.
>>
>>> On Sun, 4 Feb 2024, Song, Xiongwei wrote:
>>>
>>>> Once SLAB_MEM_SPREAD is removed, IMO, cpuset.memory_spread_slab is 
>>>> useless.
>>> SLAB_MEM_SPREAD does not do anything anymore. SLUB relies on the
>>> "spreading" via the page allocator memory policies instead of doing its
>>> own like SLAB used to do.
>>>
>>> What does FILE_SPREAD_SLAB do? Dont see anything there either.
>> The FILE_SPREAD_SLAB flag is used by cpuset.memory_spread_slab with 
>> read/write operations:
>>
>> In kernel/cgroup/cpuset.c,
>> static struct cftype legacy_files[] = {
>> ... snip ...
>>          {
>>                  .name = "memory_spread_slab",
>>                  .read_u64 = cpuset_read_u64,
>>                  .write_u64 = cpuset_write_u64,
>>                  .private = FILE_SPREAD_SLAB,
>>          },
>> ... snip ...
>> };
>
> It looks like that memory_spread_slab may have effect only on the slab 
> allocator. With the removal of the slab allocator, memory_spread_slab 
> is now a no-op. However, the memory_spread_slab cgroupfs file is an 
> externally visible API. So we can't just remove it as it may break 
> existing applications. We can certainly deprecate it and advise users 
> not to use it.

BTW, cpuset doesn't use SLAB_MEM_SPREAD directly. Instead it set the 
task's PFA_SPREAD_SLAB and let other subsystems test it to act 
appropriately. Other than cpuset, the latest upstream kernel doesn't 
check or use this flag at all.

Cheers,
Longman