Re: [PATCH v2 24/27] function_graph: Use static_call and branch to optimize entry function

Masami Hiramatsu
2024-06-03T03:11:13Z

On Sat, 01 Jun 2024 23:38:08 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
>
> In most cases function graph is used by a single user. Instead of calling
> a loop to call function graph callbacks in this case, call the function
> entry callback directly.
>
> Add a static_key that will be used to set the function graph logic to
> either do the loop (when more than one callback is registered) or to call
> the callback directly if there is only one registered callback.

I understand this works, but my concern is that, if we use fprobe
and function_graph at the same time, does it always loop on both gops?

I mean if those are the subops of one ftrace_ops, ftrace_trampoline
will always call the same function_graph_enter() for both gops, and loop
on the gops list.

For example, if there are 2 fgraph_ops, one has "vfs_*" filter and
another has "sched_*" filter, those does not cover each other.

Are there any way to solve this issue? I think my previous series
calls function_graph_enter_ops() directly from trampoline (If it works
correctly...)

Thank you,

>
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
> kernel/trace/fgraph.c | 77 ++++++++++++++++++++++++++++++++++++-------
> 1 file changed, 66 insertions(+), 11 deletions(-)
>
> diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
> index 4d566a0a741d..7c3b0261b1bb 100644
> --- a/kernel/trace/fgraph.c
> +++ b/kernel/trace/fgraph.c
> @@ -11,6 +11,7 @@
> #include <linux/jump_label.h>
> #include <linux/suspend.h>
> #include <linux/ftrace.h>
> +#include <linux/static_call.h>
> #include <linux/slab.h>
>
> #include <trace/events/sched.h>
> @@ -511,6 +512,10 @@ static struct fgraph_ops fgraph_stub = {
> .retfunc = ftrace_graph_ret_stub,
> };
>
> +static struct fgraph_ops *fgraph_direct_gops = &fgraph_stub;
> +DEFINE_STATIC_CALL(fgraph_func, ftrace_graph_entry_stub);
> +DEFINE_STATIC_KEY_TRUE(fgraph_do_direct);
> +
> /**
> * ftrace_graph_stop - set to permanently disable function graph tracing
> *
> @@ -636,21 +641,34 @@ int function_graph_enter(unsigned long ret, unsigned long func,
> if (offset < 0)
> goto out;
>
> - for_each_set_bit(i, &fgraph_array_bitmask,
> - sizeof(fgraph_array_bitmask) * BITS_PER_BYTE) {
> - struct fgraph_ops *gops = fgraph_array[i];
> - int save_curr_ret_stack;
> -
> - if (gops == &fgraph_stub)
> - continue;
> +#ifdef CONFIG_HAVE_STATIC_CALL
> + if (static_branch_likely(&fgraph_do_direct)) {
> + int save_curr_ret_stack = current->curr_ret_stack;
>
> - save_curr_ret_stack = current->curr_ret_stack;
> - if (ftrace_ops_test(&gops->ops, func, NULL) &&
> - gops->entryfunc(&trace, gops))
> - bitmap |= BIT(i);
> + if (static_call(fgraph_func)(&trace, fgraph_direct_gops))
> + bitmap |= BIT(fgraph_direct_gops->idx);
> else
> /* Clear out any saved storage */
> current->curr_ret_stack = save_curr_ret_stack;
> + } else
> +#endif
> + {
> + for_each_set_bit(i, &fgraph_array_bitmask,
> + sizeof(fgraph_array_bitmask) * BITS_PER_BYTE) {
> + struct fgraph_ops *gops = fgraph_array[i];
> + int save_curr_ret_stack;
> +
> + if (gops == &fgraph_stub)
> + continue;
> +
> + save_curr_ret_stack = current->curr_ret_stack;
> + if (ftrace_ops_test(&gops->ops, func, NULL) &&
> + gops->entryfunc(&trace, gops))
> + bitmap |= BIT(i);
> + else
> + /* Clear out any saved storage */
> + current->curr_ret_stack = save_curr_ret_stack;
> + }
> }
>
> if (!bitmap)
> @@ -1155,6 +1173,8 @@ void fgraph_update_pid_func(void)
> gops = container_of(op, struct fgraph_ops, ops);
> gops->entryfunc = ftrace_pids_enabled(op) ?
> fgraph_pid_func : gops->saved_func;
> + if (ftrace_graph_active == 1)
> + static_call_update(fgraph_func, gops->entryfunc);
> }
> }
> }
> @@ -1209,6 +1229,32 @@ static void init_task_vars(int idx)
> read_unlock(&tasklist_lock);
> }
>
> +static void ftrace_graph_enable_direct(bool enable_branch)
> +{
> + trace_func_graph_ent_t func = NULL;
> + int i;
> +
> + for_each_set_bit(i, &fgraph_array_bitmask,
> + sizeof(fgraph_array_bitmask) * BITS_PER_BYTE) {
> + func = fgraph_array[i]->entryfunc;
> + fgraph_direct_gops = fgraph_array[i];
> + }
> + if (WARN_ON_ONCE(!func))
> + return;
> +
> + static_call_update(fgraph_func, func);
> + if (enable_branch)
> + static_branch_disable(&fgraph_do_direct);
> +}
> +
> +static void ftrace_graph_disable_direct(bool disable_branch)
> +{
> + if (disable_branch)
> + static_branch_disable(&fgraph_do_direct);
> + static_call_update(fgraph_func, ftrace_graph_entry_stub);
> + fgraph_direct_gops = &fgraph_stub;
> +}
> +
> int register_ftrace_graph(struct fgraph_ops *gops)
> {
> int command = 0;
> @@ -1235,7 +1281,11 @@ int register_ftrace_graph(struct fgraph_ops *gops)
>
> ftrace_graph_active++;
>
> + if (ftrace_graph_active == 2)
> + ftrace_graph_disable_direct(true);
> +
> if (ftrace_graph_active == 1) {
> + ftrace_graph_enable_direct(false);
> register_pm_notifier(&ftrace_suspend_notifier);
> ret = start_graph_tracing();
> if (ret)
> @@ -1292,6 +1342,11 @@ void unregister_ftrace_graph(struct fgraph_ops *gops)
>
> ftrace_shutdown_subops(&graph_ops, &gops->ops, command);
>
> + if (ftrace_graph_active == 1)
> + ftrace_graph_enable_direct(true);
> + else if (!ftrace_graph_active)
> + ftrace_graph_disable_direct(false);
> +
> if (!ftrace_graph_active) {
> ftrace_graph_return = ftrace_stub_graph;
> ftrace_graph_entry = ftrace_graph_entry_stub;
> --
> 2.43.0
>
>

--
Masami Hiramatsu (Google) <mhiramat@kernel.org>