Re: [PATCH 10/20] function_graph: Have the instances use their own ftrace_ops for filtering

Steven Rostedt
2024-05-31T22:49:47Z

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