linux-kernel - Re: linux-next: manual merge of the rcu tree with the ftrace tree

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Message-ID: <59631498-2013-4224-adcf-f0afe6e7ff14@linux.dev>
Date: Sun, 30 Nov 2025 10:49:36 -0800
From: Yonghong Song <yonghong.song@...ux.dev>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: paulmck@...nel.org, Steven Rostedt <rostedt@...dmis.org>,
 Stephen Rothwell <sfr@...b.auug.org.au>,
 Frederic Weisbecker <frederic@...nel.org>,
 Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
 Boqun Feng <boqun.feng@...il.com>, Uladzislau Rezki <urezki@...il.com>,
 Masami Hiramatsu <mhiramat@...nel.org>,
 Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
 Linux Next Mailing List <linux-next@...r.kernel.org>
Subject: Re: linux-next: manual merge of the rcu tree with the ftrace tree



On 11/17/25 11:35 PM, Sebastian Andrzej Siewior wrote:
> On 2025-11-14 10:31:45 [-0800], Yonghong Song wrote:
>>> I believe that migration needs to be disabled at this point, but I am
>>> again adding Yonghong on CC for his perspective.
>> Yes, migration needs to be disabled for rt kernel in order to let
>> bpf program running properly.
> Why is disabling migration special in regard to RT kernels vs !RT?
> Why do we need to disable migration given that bpf_prog_run_array()
> already does that? Is there a different entry point?

bpf_prog_run_array() has two callers. One is trace_call_bpf() in
kernel/trace/bpf_trace.c, and the other is lirc_bpf_run() in
drivers/media/rc/bpf-lirc.c. The migration disable/enabled is
needed for lirc_bpf_run().

> My point why is it required to disable migration on trace-point entry
> for BPF given that the BPF-entry already does so.

In trace_call_bpf(), we have
    if (unlikely(__this_cpu_inc_return(bpf_prog_active) != 1)) { ... }
So migriate_disable() is necessary.

>
> Sebastian