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@igalia.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();
+}
+
 __bpf_kfunc_end_defs();
 
 static void bpf_task_work_cancel_scheduled(struct irq_work *irq_work)
@@ -4546,6 +4586,10 @@ BTF_ID_FLAGS(func, bpf_task_work_schedule_signal, KF_IMPLICIT_ARGS)
 BTF_ID_FLAGS(func, bpf_task_work_schedule_resume, KF_IMPLICIT_ARGS)
 BTF_ID_FLAGS(func, bpf_dynptr_from_file)
 BTF_ID_FLAGS(func, bpf_dynptr_file_discard)
+BTF_ID_FLAGS(func, bpf_in_nmi)
+BTF_ID_FLAGS(func, bpf_in_hardirq)
+BTF_ID_FLAGS(func, bpf_in_serving_softirq)
+BTF_ID_FLAGS(func, bpf_in_task)
 BTF_KFUNCS_END(common_btf_ids)
 
 static const struct btf_kfunc_id_set common_kfunc_set = {
-- 
2.52.0

On Fri, Jan 23, 2026 at 12:43 AM Changwoo Min <changwoo@igalia.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@igalia.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.
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().

pw-bot: cr

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@igalia.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@igalia.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