Re: [RFC PATCH v4 2/3] trace: integrate stackmap into ftrace stack recording path

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 16 Jun 2026 14:41:18 +0800
Li Pengfei <ljdlns1987@gmail.com> wrote:

>  int set_tracer_flag(struct trace_array *tr, u64 mask, int enabled)
>  {
>  	switch (mask) {
> @@ -3993,6 +4091,33 @@ int set_tracer_flag(struct trace_array *tr, u64 mask, int enabled)
>  	if (!!(tr->trace_flags & mask) == !!enabled)
>  		return 0;
>  
> +#ifdef CONFIG_FTRACE_STACKMAP
> +	/*
> +	 * STACKMAP is intentionally global-instance-only: the dedup map,
> +	 * its tracefs files (stack_map / stack_map_stat / stack_map_bin)
> +	 * and the lifetime/reset semantics are tied to the global trace
> +	 * array. options/stackmap is hidden on secondary instances via
> +	 * TOP_LEVEL_TRACE_FLAGS, but writes still reach set_tracer_flag()
> +	 * through the aggregate trace_options file. Reject the enable on
> +	 * a secondary instance so it cannot be silently accepted and then
> +	 * become a no-op in the hot path (where tr->stackmap is NULL and
> +	 * the code falls back to a full stack trace).
> +	 *
> +	 * On the global instance, allow the enable while init is still
> +	 * pending (boot-time trace_options=stackmap is applied before the
> +	 * tracefs init work creates the map; the hot path falls back
> +	 * until the map is published). Only reject once init has
> +	 * permanently failed, so options/stackmap never reports an
> +	 * enabled no-op. READ_ONCE() suffices: this only inspects the
> +	 * init state, it does not dereference the map (the hot path uses
> +	 * smp_load_acquire(&tr->stackmap) for that).
> +	 */
> +	if (mask == TRACE_ITER(STACKMAP) && enabled &&
> +	    (tr != &global_trace ||
> +	     READ_ONCE(stackmap_init_state) == STACKMAP_INIT_FAILED))
> +		return -EINVAL;
> +#endif
> +
>  	/* Give the tracer a chance to approve the change */
>  	if (tr->current_trace->flag_changed)
>  		if (tr->current_trace->flag_changed(tr, mask, !!enabled))
> @@ -9222,6 +9347,91 @@ static __init void tracer_init_tracefs_work_func(struct work_struct *work)
>  			NULL, &tracing_dyn_info_fops);
>  #endif
>  
> +#ifdef CONFIG_FTRACE_STACKMAP
> +	{
> +		struct ftrace_stackmap *smap;
> +		struct dentry *map_file;
> +
> +		smap = ftrace_stackmap_create(&global_trace);
> +		if (!IS_ERR(smap)) {
> +			/*
> +			 * Failure-atomic init: stack_map is the single
> +			 * required tracefs file (it doubles as the reset
> +			 * interface and the human-readable resolver). If
> +			 * we cannot create it, the hot path must not be
> +			 * able to emit <stack_id N> events that no one can
> +			 * resolve or clear, so refuse to publish the map
> +			 * and tear it down.
> +			 *
> +			 * Create stack_map BEFORE smp_store_release() so an
> +			 * observed non-NULL global_trace.stackmap implies
> +			 * its resolver/reset file exists.
> +			 */
> +			map_file = trace_create_file("stack_map",
> +						     TRACE_MODE_WRITE, NULL,
> +						     smap,
> +						     &ftrace_stackmap_fops);
> +			if (!map_file) {
> +				pr_warn("ftrace stackmap init: stack_map create failed, dedup disabled\n");
> +				ftrace_stackmap_destroy(smap);
> +				/*
> +				 * Permanent failure. Record it and clear a
> +				 * STACKMAP flag that a boot-time
> +				 * trace_options=stackmap may have set, so
> +				 * options/stackmap does not report an
> +				 * enabled no-op and later userspace enables
> +				 * return -EINVAL.
> +				 */
> +				WRITE_ONCE(stackmap_init_state,
> +					   STACKMAP_INIT_FAILED);
> +				global_trace.trace_flags &=
> +					~TRACE_ITER(STACKMAP);

80 columns is no longer a hard requirement. 100 is more the default, so the
above should be:

				WRITE_ONCE(stackmap_init_state, STACKMAP_INIT_FAILED);
				global_trace.trace_flags &= ~TRACE_ITER(STACKMAP);



> +			} else {
> +				/*
> +				 * smp_store_release pairs with the
> +				 * smp_load_acquire() in
> +				 * __ftrace_trace_stack(). Publishing only
> +				 * after the required file exists keeps
> +				 * "smap visible" => "resolver/reset
> +				 * available".
> +				 */

> +				smp_store_release(&global_trace.stackmap,
> +						  smap);
> +				WRITE_ONCE(stackmap_init_state,
> +					   STACKMAP_INIT_DONE);

Same with the above two.

> +				/*
> +				 * stat and bin are auxiliary observability
> +				 * surfaces. If they fail to be created we
> +				 * keep dedup enabled (the kernel side still
> +				 * works, and stack_map alone is enough to
> +				 * resolve and reset); trace_create_file()
> +				 * already pr_warn()s on failure.
> +				 */
> +				trace_create_file("stack_map_stat",
> +						  TRACE_MODE_READ, NULL,
> +						  smap,
> +						  &ftrace_stackmap_stat_fops);
> +				trace_create_file("stack_map_bin",
> +						  TRACE_MODE_READ, NULL,
> +						  smap,
> +						  &ftrace_stackmap_bin_fops);
> +			}

-- Steve