linux-kernel - Re: [PATCH bpf-next 1/3] bpf, x64: Call perf_snapshot_branch

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Message-ID: <216a9728-ae69-43af-8632-471b71c56607@linux.dev>
Date: Sat, 10 Jan 2026 00:31:20 +0800
From: Leon Hwang <leon.hwang@...ux.dev>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: bpf <bpf@...r.kernel.org>, 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>, "David S . Miller" <davem@...emloft.net>,
 David Ahern <dsahern@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>, X86 ML <x86@...nel.org>,
 "H . Peter Anvin" <hpa@...or.com>, Matt Bobrowski
 <mattbobrowski@...gle.com>, Steven Rostedt <rostedt@...dmis.org>,
 Masami Hiramatsu <mhiramat@...nel.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Shuah Khan <shuah@...nel.org>, Network Development <netdev@...r.kernel.org>,
 LKML <linux-kernel@...r.kernel.org>,
 linux-trace-kernel <linux-trace-kernel@...r.kernel.org>,
 "open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>,
 kernel-patches-bot@...com
Subject: Re: [PATCH bpf-next 1/3] bpf, x64: Call perf_snapshot_branch_stack in
 trampoline

On 2026/1/10 00:24, Alexei Starovoitov wrote:
> On Fri, Jan 9, 2026 at 7:37 AM Leon Hwang <leon.hwang@...ux.dev> wrote:
>>

[...]

>> @@ -3366,6 +3416,14 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *rw_im
>>
>>         save_args(m, &prog, regs_off, false, flags);
>>
>> +       if (bpf_prog_copy_branch_snapshot(fentry)) {
>> +               /* Get branch snapshot asap. */
>> +               if (invoke_branch_snapshot(&prog, image, rw_image)) {
>> +                       ret = -EINVAL;
>> +                       goto cleanup;
>> +               }
>> +       }
> 
> Andrii already tried to do it.
> I hated it back then and still hate the idea.
> We're not going to add custom logic for one specific use case
> no matter how appealing it sounds to save very limited LBR entries.
> The HW will get better, but we will be stuck with this optimization forever.
> 

Understood, thanks for the explanation.

I won’t pursue this approach further.

Thanks,
Leon