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

From: Sebastian Andrzej Siewior <[email-masked]>
Date: Fri, 14 Nov 2025 17:33:30 +0100

On 2025-11-14 11:22:02 [-0500], Steven Rostedt wrote:
> It's to match this code:
> 
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -100,6 +100,25 @@ void for_each_tracepoint_in_module(struct module *mod,
>  }
>  #endif /* CONFIG_MODULES */
>  
> +/*
> + * BPF programs can attach to the tracepoint callbacks. But if the
> + * callbacks are called with preemption disabled, the BPF programs
> + * can cause quite a bit of latency. When PREEMPT_RT is enabled,
> + * instead of disabling preemption, use srcu_fast_notrace() for
> + * synchronization. As BPF programs that are attached to tracepoints
> + * expect to stay on the same CPU, also disable migration.
> + */
> +#ifdef CONFIG_PREEMPT_RT
> +extern struct srcu_struct tracepoint_srcu;
> +# define tracepoint_sync() synchronize_srcu(&tracepoint_srcu);
> +# define tracepoint_guard()                            \
> +       guard(srcu_fast_notrace)(&tracepoint_srcu);     \
> +       guard(migrate)()
> +#else
> +# define tracepoint_sync() synchronize_rcu();
> +# define tracepoint_guard() guard(preempt_notrace)()
> +#endif
> +
> 
> Where in PREEMPT_RT we do not disable preemption around the tracepoint
> callback, but in non RT we do. Instead it uses a srcu and migrate disable.

I appreciate the effort. I really do. But why can't we have SRCU on both
configs?

Also why does tracepoint_guard() need to disable migration? The BPF
program already disables migrations (see for instance
bpf_prog_run_array()).
This is true for RT and !RT. So there is no need to do it here.

> The migrate_disable in the syscall tracepoint (which gets called by the
> system call version that doesn't disable migration, even in RT), needs to
> disable migration so that the accounting that happens in:
> 
>   trace_event_buffer_reserve()
> 
> matches what happens when that function gets called by a normal tracepoint
> callback.

buh. But this is something. If we know that the call chain does not
disable migration, couldn't we just use a different function? I mean we
have tracing_gen_ctx_dec() and tracing_gen_ctx)(). Wouldn't this work
for migrate_disable(), too? 
Just in case we need it and can not avoid it, see above.

> -- Steve

Sebastian