Message-ID: <6c9678a8-2f1a-44ea-a28d-36a1ab0ad763@igalia.com>
Date: Sun, 25 Jan 2026 20:12:14 +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 v2 2/2] selftests/bpf: Add tests for execution
 context helpers

Hi Alexei,

Thank you for the review.

On 1/25/26 12:37 PM, Alexei Starovoitov wrote:
> On Sat, Jan 24, 2026 at 5:27 AM Changwoo Min <changwoo@...lia.com> wrote:
>>
>> Add a new selftest suite `exe_ctx` to verify the accuracy of the
>> bpf_in_task(), bpf_in_hardirq(), and bpf_in_serving_softirq() helpers
>> introduced in bpf_experimental.h.
>>
>> Testing these execution contexts deterministically requires crossing
>> context boundaries within a single CPU. To achieve this, the test
>> implements a "Trigger-Observer" pattern using bpf_testmod:
>>
>> 1. Trigger: A BPF syscall program calls a new bpf_testmod kfunc
>>     bpf_kfunc_trigger_ctx_check().
>> 2. Task to HardIRQ: The kfunc uses irq_work_queue() to trigger a
>>     self-IPI on the local CPU.
>> 3. HardIRQ to SoftIRQ: The irq_work handler calls a dummy function
>>     (observed by BPF fentry) and then schedules a tasklet to
>>     transition into SoftIRQ context.
>>
>> The user-space runner ensures determinism by pinning itself to CPU 0
>> before execution, forcing the entire interrupt chain to remain on a
>> single core. Dummy noinline functions with compiler barriers are
>> added to bpf_testmod.c to serve as stable attachment points for
>> fentry programs. A retry loop is used in user-space to wait for the
>> asynchronous SoftIRQ to complete.
>>
>> Signed-off-by: Changwoo Min <changwoo@...lia.com>
> 
> ...
> 
>> +#include "vmlinux.h"
>> +#include <bpf/bpf_helpers.h>
>> +#include <bpf/bpf_tracing.h>
>> +#include "bpf_experimental.h"
>> +
>> +char _license[] SEC("license") = "GPL";
>> +
>> +extern void bpf_kfunc_trigger_ctx_check(void) __ksym;
>> +
>> +int count_hardirq;
>> +int count_softirq;
>> +int count_task;
>> +
>> +/* Triggered via bpf_prog_test_run from user-space */
>> +SEC("syscall")
>> +int trigger_all_contexts(void *ctx)
>> +{
>> +       if (bpf_in_task())
>> +               __sync_fetch_and_add(&count_task, 1);
>> +
>> +       /* Trigger the firing of a hardirq and softirq for test. */
>> +       bpf_kfunc_trigger_ctx_check();
>> +       return 0;
>> +}
>> +
>> +/* Observer for HardIRQ */
>> +SEC("fentry/bpf_testmod_test_hardirq_fn")
>> +int BPF_PROG(on_hardirq)
>> +{
>> +       if (bpf_in_hardirq())
>> +               __sync_fetch_and_add(&count_hardirq, 1);
>> +       return 0;
>> +}
>> +
>> +/* Observer for SoftIRQ */
>> +SEC("fentry/bpf_testmod_test_softirq_fn")
>> +int BPF_PROG(on_softirq)
>> +{
>> +       if (bpf_in_serving_softirq())
>> +               __sync_fetch_and_add(&count_softirq, 1);
>> +       return 0;
>> +}
>> diff --git a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
>> index d425034b72d3..1b04022859b7 100644
>> --- a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
>> +++ b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
>> @@ -1164,6 +1164,33 @@ __bpf_kfunc int bpf_kfunc_implicit_arg(int a, struct bpf_prog_aux *aux);
>>   __bpf_kfunc int bpf_kfunc_implicit_arg_legacy(int a, int b, struct bpf_prog_aux *aux);
>>   __bpf_kfunc int bpf_kfunc_implicit_arg_legacy_impl(int a, int b, struct bpf_prog_aux *aux);
>>
>> +/* hook targets */
>> +noinline void bpf_testmod_test_hardirq_fn(void) { barrier(); }
>> +noinline void bpf_testmod_test_softirq_fn(void) { barrier(); }
>> +
>> +/* Tasklet for SoftIRQ context */
>> +static void ctx_check_tasklet_fn(struct tasklet_struct *t)
>> +{
>> +       bpf_testmod_test_softirq_fn();
>> +}
>> +
>> +DECLARE_TASKLET(ctx_check_tasklet, ctx_check_tasklet_fn);
>> +
>> +/* IRQ Work for HardIRQ context */
>> +static void ctx_check_irq_fn(struct irq_work *work)
>> +{
>> +       bpf_testmod_test_hardirq_fn();
>> +       tasklet_schedule(&ctx_check_tasklet);
>> +}
>> +
>> +static struct irq_work ctx_check_irq = IRQ_WORK_INIT_HARD(ctx_check_irq_fn);
> 
> Nicely done! selftests should work in PREEMPT_RT too
> though we don't enable it in bpf CI.
> 
> I was about to apply it, but the new tests fails on s390:
> 
> test_exe_ctx:FAIL:hardirq_ok unexpected hardirq_ok: actual 0 <= expected 0
> test_exe_ctx:FAIL:softirq_ok unexpected softirq_ok: actual 0 <= expected 0
> 
> The existing bpf_in_interrupt() also works on x86 and arm64 only.
> When it was introduced it came with pretty weak selftest
> commit 31329b6 ("selftests/bpf: Introduce experimental bpf_in_interrupt()")
> so it's not really testing anything on s390 and non-x86/arm architectures.
> 
> So just add your strong selftests to DENYLIST.s390x.
> get_preempt_count() on s390 looks like this:
> static __always_inline int preempt_count(void)
> {
>     return READ_ONCE(get_lowcore()->preempt_count) & ~PREEMPT_NEED_RESCHED;
> }
> but get_lowcore() needs asm.
> So it's not going to be easy to make it work purely in bpf.
> Let's punt it to people that care about s390.


Regarding the s390 failure: that makes sense. Since the
get_preempt_count() logic relies on architecture-specific register/
memory access that hasn't been implemented for s390 BPF yet, I will add
exe_ctx to DENYLIST.s390x in v3.

I'll send out the v3 with the denylist update shortly. Also, I will add
comments to those helpers mentioning that only x86 and arm64 are
supported.

Regards,
Changwoo

> 
> pw-bot: cr
> 

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