linux-kernel - Re: [PATCH] ftrace: Avoid needless updates of the ftrace function call

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Date:   Tue, 22 Nov 2022 22:17:11 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Song Shuai <suagrfillet@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH] ftrace: Avoid needless updates of the ftrace function
 call

On Wed, 23 Nov 2022 10:29:28 +0800
Song Shuai <suagrfillet@...il.com> wrote:

> > @@ -2783,7 +2796,7 @@ void ftrace_modify_all_code(int command)
> >          * traced.
> >          */
> >         if (update) {
> > -               err = ftrace_update_ftrace_func(ftrace_ops_list_func);
> > +               err = update_ftrace_func(ftrace_ops_list_func);
> >                 if (FTRACE_WARN_ON(err))
> >                         return;
> >         }
> > @@ -2799,7 +2812,7 @@ void ftrace_modify_all_code(int command)
> >                 /* If irqs are disabled, we are in stop machine */
> >                 if (!irqs_disabled())
> >                         smp_call_function(ftrace_sync_ipi, NULL, 1);
> > -               err = ftrace_update_ftrace_func(ftrace_trace_function);
> > +               err = update_ftrace_func(ftrace_trace_function);  
> The helper function should only serve the ftrace_ops_list_func.
> 
> And ftrace_trace_function is always different in this function with
> FTRACE_UPDATE_TRACE_FUNC command.
> 
> So this place should be left as it is.

But it is needed to update the static variable. Without this change,
then save_func will get set to ftrace_ops_list_func and never change
after that and then the update to ftrace_ops_list_func will stop happening.

-- Steve