linux-kernel - Re: [PATCH] selftests/bpf: Make x86 preempt

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Message-ID: <d41d48a9-f2f7-49fa-82bc-5b8237246c7b@igalia.com>
Date: Fri, 30 Jan 2026 10:10:56 +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] selftests/bpf: Make x86 preempt_count access compatible
 across v6.14+

Thank you, Alexei, for quick review.

On 1/30/26 2:21 AM, Alexei Starovoitov wrote:
> On Thu, Jan 29, 2026 at 5:54 AM Changwoo Min <changwoo@...lia.com> wrote:
>>
>> Recent x86 kernels (v6.15+) export __preempt_count as a ksym, while older
>> kernels expose the preemption counter via pcpu_hot.preempt_count. The
>> existing selftest helper unconditionally dereferenced __preempt_count,
>> which breaks BPF program loading on older kernels.
>>
>> Make the x86 preemption count lookup version-agnostic by:
>> - Marking __preempt_count and pcpu_hot as weak ksyms.
>> - Introducing a BTF-described pcpu_hot___local layout with
>>    preserve_access_index.
>> - Selecting the appropriate access path at runtime using ksym availability
>>    and bpf_core_field_exists().
>>
>> This allows a single BPF binary to run correctly on both v6.14-and-older
>> and v6.15-and-newer kernels without relying on compile-time version checks.
> 
> See.. with bpf approach instead of kfunc this new helpers
> can work on old kernels without backporting kfuncs :)

You are right. I love the flexibility of BPF in deployment! \o/

> 
>> Fixes: 4b69e31329b6 ("selftests/bpf: Introduce experimental bpf_in_interrupt()")
> 
> fixes tag is not appropriate. It's not a bug fix.

Sure, I will drop the Fixes tag.

> 
>> Signed-off-by: Changwoo Min <changwoo@...lia.com>
>> ---
>>   tools/testing/selftests/bpf/bpf_experimental.h | 16 ++++++++++++++--
>>   1 file changed, 14 insertions(+), 2 deletions(-)
>>
>> diff --git a/tools/testing/selftests/bpf/bpf_experimental.h b/tools/testing/selftests/bpf/bpf_experimental.h
>> index a39576c8ba04..0194c0090e50 100644
>> --- a/tools/testing/selftests/bpf/bpf_experimental.h
>> +++ b/tools/testing/selftests/bpf/bpf_experimental.h
>> @@ -614,7 +614,13 @@ extern int bpf_cgroup_read_xattr(struct cgroup *cgroup, const char *name__str,
>>
>>   extern bool CONFIG_PREEMPT_RT __kconfig __weak;
>>   #ifdef bpf_target_x86
>> -extern const int __preempt_count __ksym;
>> +extern const int __preempt_count __ksym __weak;
>> +
>> +struct pcpu_hot___local {
>> +       int preempt_count;
>> +} __attribute__((preserve_access_index));
>> +
>> +extern struct pcpu_hot___local pcpu_hot __ksym __weak;
>>   #endif
>>
>>   struct task_struct___preempt_rt {
>> @@ -624,7 +630,13 @@ struct task_struct___preempt_rt {
>>   static inline int get_preempt_count(void)
>>   {
>>   #if defined(bpf_target_x86)
>> -       return *(int *) bpf_this_cpu_ptr(&__preempt_count);
>> +       /* v6.15 or later */
>> +       if (&__preempt_count)
>> +               return *(int *) bpf_this_cpu_ptr(&__preempt_count);
> 
> please use bpf_ksym_exists().
> It helps to catch missing __weak. This patch adds it,
> but let's demonstrate best coding practices.

Sure, will change it as suggested.


> 
>> +       /* v6.14 or older */
>> +       if (bpf_core_field_exists(pcpu_hot.preempt_count))
>> +               return ((struct pcpu_hot___local *)
>> +                       bpf_this_cpu_ptr(&pcpu_hot))->preempt_count;
> 
> iirc pcpu_hot approach was there for a short time.
> Like 5.x kernel didn't have it. It was per-cpu var too.
> Pls adjust the comment.

Sure, I found that pcpu_hot had been used only between 6.1 -- 6.14.

I will send out v2 shortly.
Regards,
Changwoo Min

> 
> pw-bot: cr
>