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Date: Fri, 31 May 2024 18:49:10 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, Mark
 Rutland <mark.rutland@arm.com>, Mathieu Desnoyers
 <mathieu.desnoyers@efficios.com>, Andrew Morton
 <akpm@linux-foundation.org>, Alexei Starovoitov
 <alexei.starovoitov@gmail.com>, Florent Revest <revest@chromium.org>,
 Martin KaFai Lau <martin.lau@linux.dev>, bpf <bpf@vger.kernel.org>, Sven
 Schnelle <svens@linux.ibm.com>, Alexei Starovoitov <ast@kernel.org>, Jiri
 Olsa <jolsa@kernel.org>, Arnaldo Carvalho de Melo <acme@kernel.org>, Daniel
 Borkmann <daniel@iogearbox.net>, Alan Maguire <alan.maguire@oracle.com>,
 Peter Zijlstra <peterz@infradead.org>, Thomas Gleixner
 <tglx@linutronix.de>, Guo Ren <guoren@kernel.org>
Subject: Re: [PATCH 10/20] function_graph: Have the instances use their own
 ftrace_ops for filtering
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In-Reply-To: <20240531235023.a0b2b207362eba2f8b5c16f7@kernel.org>
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On Fri, 31 May 2024 23:50:23 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> So is it similar to the fprobe/kprobe, use shared signle ftrace_ops,
> but keep each fgraph has own hash table?

Sort of.

I created helper functions in ftrace that lets you have a "manager
ftrace_ops" that will be used to assign to ftrace (with the function
that will demultiplex), and then you can have "subops" that can be
assigned that is per user. Here's a glimpse of the code:

/**
 * ftrace_startup_subops - enable tracing for subops of an ops
 * @ops: Manager ops (used to pick all the functions of its subops)
 * @subops: A new ops to add to @ops
 * @command: Extra commands to use to enable tracing
 *
 * The @ops is a manager @ops that has the filter that includes all the functions
 * that its list of subops are tracing. Adding a new @subops will add the
 * functions of @subops to @ops.
 */
int ftrace_startup_subops(struct ftrace_ops *ops, struct ftrace_ops *subops, int command)
{
	struct ftrace_hash *filter_hash;
	struct ftrace_hash *notrace_hash;
	struct ftrace_hash *save_filter_hash;
	struct ftrace_hash *save_notrace_hash;
	int size_bits;
	int ret;

	if (unlikely(ftrace_disabled))
		return -ENODEV;

	ftrace_ops_init(ops);
	ftrace_ops_init(subops);

	/* Make everything canonical (Just in case!) */
	if (!ops->func_hash->filter_hash)
		ops->func_hash->filter_hash = EMPTY_HASH;
	if (!ops->func_hash->notrace_hash)
		ops->func_hash->notrace_hash = EMPTY_HASH;
	if (!subops->func_hash->filter_hash)
		subops->func_hash->filter_hash = EMPTY_HASH;
	if (!subops->func_hash->notrace_hash)
		subops->func_hash->notrace_hash = EMPTY_HASH;

	/* For the first subops to ops just enable it normally */
	if (list_empty(&ops->subop_list)) {
		/* Just use the subops hashes */
		filter_hash = copy_hash(subops->func_hash->filter_hash);
		notrace_hash = copy_hash(subops->func_hash->notrace_hash);
		if (!filter_hash || !notrace_hash) {
			free_ftrace_hash(filter_hash);
			free_ftrace_hash(notrace_hash);
			return -ENOMEM;
		}

		save_filter_hash = ops->func_hash->filter_hash;
		save_notrace_hash = ops->func_hash->notrace_hash;

		ops->func_hash->filter_hash = filter_hash;
		ops->func_hash->notrace_hash = notrace_hash;
		list_add(&subops->list, &ops->subop_list);
		ret = ftrace_startup(ops, command);
		if (ret < 0) {
			list_del(&subops->list);
			ops->func_hash->filter_hash = save_filter_hash;
			ops->func_hash->notrace_hash = save_notrace_hash;
			free_ftrace_hash(filter_hash);
			free_ftrace_hash(notrace_hash);
		} else {
			free_ftrace_hash(save_filter_hash);
			free_ftrace_hash(save_notrace_hash);
			subops->flags |= FTRACE_OPS_FL_ENABLED;
		}
		return ret;
	}

	/*
	 * Here there's already something attached. Here are the rules:
	 *   o If either filter_hash is empty then the final stays empty
	 *      o Otherwise, the final is a superset of both hashes
	 *   o If either notrace_hash is empty then the final stays empty
	 *      o Otherwise, the final is an intersection between the hashes
	 */
	if (ops->func_hash->filter_hash == EMPTY_HASH ||
	    subops->func_hash->filter_hash == EMPTY_HASH) {
		filter_hash = EMPTY_HASH;
	} else {
		size_bits = max(ops->func_hash->filter_hash->size_bits,
				subops->func_hash->filter_hash->size_bits);
		filter_hash = alloc_and_copy_ftrace_hash(size_bits, ops->func_hash->filter_hash);
		if (!filter_hash)
			return -ENOMEM;
		ret = append_hash(&filter_hash, subops->func_hash->filter_hash);
		if (ret < 0) {
			free_ftrace_hash(filter_hash);
			return ret;
		}
	}

	if (ops->func_hash->notrace_hash == EMPTY_HASH ||
	    subops->func_hash->notrace_hash == EMPTY_HASH) {
		notrace_hash = EMPTY_HASH;
	} else {
		size_bits = max(ops->func_hash->filter_hash->size_bits,
				subops->func_hash->filter_hash->size_bits);
		notrace_hash = alloc_ftrace_hash(size_bits);
		if (!notrace_hash) {
			free_ftrace_hash(filter_hash);
			return -ENOMEM;
		}

		ret = intersect_hash(&notrace_hash, ops->func_hash->filter_hash,
				     subops->func_hash->filter_hash);
		if (ret < 0) {
			free_ftrace_hash(filter_hash);
			free_ftrace_hash(notrace_hash);
			return ret;
		}
	}

	list_add(&subops->list, &ops->subop_list);

	ret = ftrace_update_ops(ops, filter_hash, notrace_hash);
	free_ftrace_hash(filter_hash);
	free_ftrace_hash(notrace_hash);
	if (ret < 0)
		list_del(&subops->list);
	return ret;
}

/**
 * ftrace_shutdown_subops - Remove a subops from a manager ops
 * @ops: A manager ops to remove @subops from
 * @subops: The subops to remove from @ops
 * @command: Any extra command flags to add to modifying the text
 *
 * Removes the functions being traced by the @subops from @ops. Note, it
 * will not affect functions that are being traced by other subops that
 * still exist in @ops.
 *
 * If the last subops is removed from @ops, then @ops is shutdown normally.
 */
int ftrace_shutdown_subops(struct ftrace_ops *ops, struct ftrace_ops *subops, int command)
{
	struct ftrace_hash *filter_hash;
	struct ftrace_hash *notrace_hash;
	int ret;

	if (unlikely(ftrace_disabled))
		return -ENODEV;

	list_del(&subops->list);

	if (list_empty(&ops->subop_list)) {
		/* Last one, just disable the current ops */

		ret = ftrace_shutdown(ops, command);
		if (ret < 0) {
			list_add(&subops->list, &ops->subop_list);
			return ret;
		}

		subops->flags &= ~FTRACE_OPS_FL_ENABLED;

		free_ftrace_hash(ops->func_hash->filter_hash);
		free_ftrace_hash(ops->func_hash->notrace_hash);
		ops->func_hash->filter_hash = EMPTY_HASH;
		ops->func_hash->notrace_hash = EMPTY_HASH;

		return 0;
	}

	/* Rebuild the hashes without subops */
	filter_hash = append_hashes(ops);
	notrace_hash = intersect_hashes(ops);
	if (!filter_hash || !notrace_hash) {
		free_ftrace_hash(filter_hash);
		free_ftrace_hash(notrace_hash);
		list_add(&subops->list, &ops->subop_list);
		return -ENOMEM;
	}

	ret = ftrace_update_ops(ops, filter_hash, notrace_hash);
	if (ret < 0)
		list_add(&subops->list, &ops->subop_list);
	free_ftrace_hash(filter_hash);
	free_ftrace_hash(notrace_hash);
	return ret;
}


> 
> > This removes the need to touch the architecture code. It can also be
> > used by fprobes to handle the attachments to functions for several
> > different sets of callbacks.
> > 
> > I'll send out patches soon.  
> 
> OK, I'll wait for that.

I'm just cleaning it up. I'll post it tomorrow (your today).

-- Steve