Re: [PATCH bpf-next v1 2/3] selftests/bpf: Simplify task_local_data memory allocation

[Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>]

Amery Hung <ameryhung@gmail.com> writes:

> On Fri, Mar 27, 2026 at 9:36 AM Mykyta Yatsenko
> <mykyta.yatsenko5@gmail.com> wrote:
>>
>> Amery Hung <ameryhung@gmail.com> writes:
>>
>> > Simplify data allocation by always using aligned_alloc() and passing
>> > size_pot, size rounded up to the closest power of two to alignment.
>> >
>> > Currently, aligned_alloc(page_size, size) is only intended to be used
>> > with memory allocators that can fulfill the request without rounding
>> > size up to page_size to conserve memory. This is enabled by defining
>> > TLD_DATA_USE_ALIGNED_ALLOC. The reason to align to page_size is due to
>> > the limitation of UPTR where only a page can be pinned to the kernel.
>> > Otherwise, malloc(size * 2) is used to allocate memory for data.
>> >
>> > However, we don't need to call aligned_alloc(page_size, size) to get
>> > a contiguous memory of size bytes within a page. aligned_alloc(size_pot,
>> > ...) will also do the trick. Therefore, just use aligned_alloc(size_pot,
>> > ...) universally.
>> >
>> > As for the size argument, create a new option,
>> > TLD_DONT_ROUND_UP_DATA_SIZE, to specify not rounding up the size.
>> > This preserves the current TLD_DATA_USE_ALIGNED_ALLOC behavior, allowing
>> > memory allocators with low overhead aligned_alloc() to not waste memory.
>> > To enable this, users need to make sure it is not an undefined behavior
>> > for the memory allocator to have size not being an integral multiple of
>> > alignment.
>> Why not simplify this further and just mmap() a page?
>>
>>   data = mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE,
>>               MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>>
>> That gives you a page-aligned, page-sized buffer with no ambiguity
>> about alignment, page boundary crossings, or aligned_alloc() POSIX
>> compliance. munmap() on cleanup instead of free().
>>
>> The whole power-of-two rounding and TLD_DONT_ROUND_UP_DATA_SIZE
>> option adds complexity that's hard to justify for selftest
>> infrastructure. How does a user decide when to define/undefine
>> TLD_DONT_ROUND_UP? This is a selftest library -- the common case
>> should just work with the simplest possible approach
>
> This library, while residing in selftest, is designed to be used in
> production projects. sched_ext is in the process of adopting it.
> Therefore, simplicity is traded for efficiency.
>
> data here is allocated for every thread that calls tld_get_data(). So
> if we mmap a page for every thread (which could be in 100k order), it
> is going to waste a significant amount of memory.
>
Thanks for providing this context, it makes sense now.
>> >
>> > Compared to the current implementation, !TLD_DATA_USE_ALIGNED_ALLOC
>> > used to always waste size-byte of memory due to malloc(size * 2).
>> > Now the worst case becomes size - 1 and the best case is 0 when the size
>> > is already a power of two.
>> >