linux-kernel - Re: [PATCH bpf-next v1 1/2] bpf: Introduce execution context detection kfuncs

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Message-ID: <933f94c8-d74c-4d01-8e55-d8dab3d79f61@igalia.com>
Date: Sat, 24 Jan 2026 20:29:20 +0900
From: Changwoo Min <changwoo@...lia.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman
 <eddyz87@...il.com>, Song Liu <song@...nel.org>,
 Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, Shuah Khan <shuah@...nel.org>,
 kernel-dev@...lia.com, bpf <bpf@...r.kernel.org>, sched-ext@...ts.linux.dev,
 LKML <linux-kernel@...r.kernel.org>,
 "open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH bpf-next v1 1/2] bpf: Introduce execution context
 detection kfuncs

Hi Alexei,

Thank you for the feedback.

On 1/24/26 1:37 AM, Alexei Starovoitov wrote:
> On Fri, Jan 23, 2026 at 12:43 AM Changwoo Min <changwoo@...lia.com> wrote:
>>
>> Introduce bpf_in_nmi(), bpf_in_hardirq(), bpf_in_serving_softirq(), and
>> bpf_in_task() kfuncs to allow BPF programs to query the current execution
>> context.
>>
>> While BPF programs can sometimes infer context based on the attach point,
>> certain programs (such as those in sched_ext) may be called from multiple
>> contexts. These kfuncs provide a reliable way for logic to branch based on
>> whether the CPU is currently handling an interrupt or executing in task
>> context.
>>
>> For example, this is particularly useful for sched_ext schedulers that need
>> to differentiate between task-to-task wake-ups and interrupt-to-task
>> wake-ups.
>>
>> As the names imply, these helpers wrap the kernel's internal in_nmi(),
>> in_hardirq(), in_serving_softirq(), and in_task() macros.
>>
>> Signed-off-by: Changwoo Min <changwoo@...lia.com>
>> ---
>>   kernel/bpf/helpers.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 44 insertions(+)
>>
>> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
>> index 637677815365..cb36bc7a80c6 100644
>> --- a/kernel/bpf/helpers.c
>> +++ b/kernel/bpf/helpers.c
>> @@ -4365,6 +4365,46 @@ __bpf_kfunc int bpf_dynptr_file_discard(struct bpf_dynptr *dynptr)
>>          return 0;
>>   }
>>
>> +/**
>> + * bpf_in_nmi - Test if the current execution context is in NMI context.
>> + *
>> + * Return: true if we are in NMI context, false otherwise.
>> + */
>> +__bpf_kfunc bool bpf_in_nmi(void)
>> +{
>> +       return in_nmi();
>> +}
>> +
>> +/**
>> + * bpf_in_hardirq - Test if the current execution context is in hard IRQ context.
>> + *
>> + * Return: true if we are in hard IRQ context, false otherwise.
>> + */
>> +__bpf_kfunc bool bpf_in_hardirq(void)
>> +{
>> +       return in_hardirq();
>> +}
>> +
>> +/**
>> + * bpf_in_serving_softirq - Test if the current execution context is in softirq context.
>> + *
>> + * Return: true if we are in softirq context, false otherwise.
>> + */
>> +__bpf_kfunc bool bpf_in_serving_softirq(void)
>> +{
>> +       return in_serving_softirq();
>> +}
>> +
>> +/**
>> + * bpf_in_task - Test if the current execution context is in task context.
>> + *
>> + * Return: true if we are in task context, false otherwise.
>> + */
>> +__bpf_kfunc bool bpf_in_task(void)
>> +{
>> +       return in_task();
>> +}
>> +
> 
> This functionality is already available.
> See bpf_in_interrupt() and get_preempt_count() in bpf_experimental.h.
> 
> No need to replicate that as kfuncs.

I see. While bpf_in_interrupt() provides a general check, certain use
cases in sched_ext require more granular differentiation -- specifically
between hardirq and softirq contexts to handle different wake-up paths
correctly.

> If something can be implemented as a bpf program it should stay as
> pure bpf code, since it's more flexible and faster this way.
> llvm together with JITs completely inline get_preempt_count().
> 

I agree. Implementing these checks as pure BPF code is more efficient
due to JIT inlining. I was initially hesitant to duplicate kernel-side
macros in BPF, but since the precedent is already established in
bpf_experimental.h, I will follow that pattern.

I will drop the kfunc implementations in Patch 1 and instead add
bpf_in_nmi(), bpf_in_hardirq(), bpf_in_serving_softirq(), and
bpf_in_task() to bpf_experimental.h by wrapping get_preempt_count().

I will also update the selftests in Patch 2 to use these new headers and
submit a v2 shortly.

Regards,
Changwoo Min