linux-kernel - Re: [PATCH bpf-next v1 08/10] bpf: Add bpf_task_work_schedule_* kfuncs with KF_IMPLICIT

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Message-ID: <3a697699-ffcb-4e2f-a7a4-9e3f571aa402@linux.dev>
Date: Mon, 12 Jan 2026 10:53:53 -0800
From: Ihor Solodrai <ihor.solodrai@...ux.dev>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman
 <eddyz87@...il.com>, Mykyta Yatsenko <yatsenko@...a.com>,
 Tejun Heo <tj@...nel.org>, Alan Maguire <alan.maguire@...cle.com>,
 Benjamin Tissoires <bentiss@...nel.org>, Jiri Kosina <jikos@...nel.org>,
 bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
 "open list:HID CORE LAYER" <linux-input@...r.kernel.org>,
 sched-ext@...ts.linux.dev
Subject: Re: [PATCH bpf-next v1 08/10] bpf: Add bpf_task_work_schedule_*
 kfuncs with KF_IMPLICIT_ARGS

On 1/9/26 1:56 PM, Ihor Solodrai wrote:
> On 1/9/26 1:49 PM, Alexei Starovoitov wrote:
>> On Fri, Jan 9, 2026 at 1:39 PM Ihor Solodrai <ihor.solodrai@...ux.dev> wrote:
>>>
>>> [...]
>>>
>>>> I feel bpf_task_work_schedule_resume() is ok to break, since it's so new.
>>>> We can remove bpf_task_work_schedule_[resume|singal]_impl()
>>>> to avoid carrying forward forever.
>>>>
>>>> bpf_stream_vprintk_impl() is not that clear. I would remove it too.
>>>
>>> That leaves only bpf_wq_set_callback_impl(). Can we break that too?
>>
>> Sounds like Benjamin is ok removing it.
>> So I think we can indeed remove them all.
>>
>>> Then there won't be legacy cases at all. It was introduced in v6.16
>>> along the with __prog suffix [1][2].
>>>
>>> If we go this route, we could clean up __prog support/docs too.
>>>
>>> I think it's worth it to make an "all or nothing" decision here:
>>> either break all 4 existing kfuncs, or backwards-support all of them.
>>
>> I don't see why "all or nothing" is a good thing.
>> It won't be "all" anyway.
>> We have bpf_rbtree_add_impl(), bpf_list_push_front_impl(), etc.
>> And those we cannot remove. sched-ext is using them.
>> Another few categories are bpf_obj_new_impl(), bpf_obj_drop_impl().
>> There are not __prog type, but conceptually the same thing and
>> KF_IMPLICIT_ARGS should support them too eventually.
> 
> I was thinking we could remove/simplify code relevant to backwards
> compat of existing _impl kfuncs. But you're right, if we start using
> implicit args for other types/kfuncs, the "legacy" case still has to
> work.
> 
> Ok, in the next revision I'll remove all the __prog users, but leave
> the "legacy" case support in place for future use.

I just had an off-list chat with Andrii, and we agreed that leaving
the existing _impl kfuncs supported may be a good idea.

It doesn't cost us much: we keep the mechanism for legacy functions
anyways, so supporting bpf_wq_set_callback_impl() and co only requires
keeping definitions in the kernel.

The only benefit of *removing* these _impl functions is that we could
clean up __prog support.

But having backwards compat seems like a better deal.
What do you think?


> 
>>
>>
>>> git tag --contains bc049387b41f | grep -v rc
>>> v6.16
>>> v6.17
>>> v6.18
>>>
>>> [1] https://lore.kernel.org/all/20250513142812.1021591-1-memxor@gmail.com/
>>> [2] https://lore.kernel.org/all/20240420-bpf_wq-v2-13-6c986a5a741f@kernel.org/
>>>
>>>
>