Re: [RFC PATCH 05/10] ftrace: Drop buffer check in function profiler callbacks

[Lai Jiangshan <laijs@cn.fujitsu.com>]

Frederic Weisbecker wrote:
> Drop the null check on hlist->hash. It is wasteful because
> we don't register the tracer if the buffer allocation failed,
> and the memory barrier in register_ftrace_graph() ensure it
> is visible to the callbacks right away.
> 

When it is on a cpu which is just came up, hlist->hash is not
allocated either.

See ftrace_profile_init().

function_profile_call(), profile_graph_entry() and profile_graph_return()
on a just online cpu are wasteful. I think we need call
ftrace_profile_init_cpu()(if ftrace_profile_enabled=1) when CPU_UP_PREPARE.

> Also we know the tracing callbacks won't be called after
> register_ftrace_graph(), so subsequent buffer resets are safe
> too.
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Li Zefan <lizf@cn.fujitsu.com>
> Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
> ---
>  kernel/trace/ftrace.c |    5 -----
>  1 files changed, 0 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 3cdb35e..dfd8f7c 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -384,8 +384,6 @@ function_profile_call(unsigned long ip, unsigned long parent_ip)
>  	local_irq_save(flags);
>  
>  	hlist = &__get_cpu_var(func_hlist_cpu);
> -	if (!hlist->hash)
> -		goto out;
>  
>  	rec = function_find_hlist_node(hlist, ip);
>  	if (!rec) {
> @@ -415,8 +413,6 @@ static void profile_graph_return(struct ftrace_graph_ret *trace)
>  
>  	local_irq_save(flags);
>  	hlist = &__get_cpu_var(func_hlist_cpu);
> -	if (!hlist->hash)
> -		goto out;
>  
>  	calltime = trace->rettime - trace->calltime;
>  
> @@ -439,7 +435,6 @@ static void profile_graph_return(struct ftrace_graph_ret *trace)
>  	if (rec)
>  		rec->time += calltime;
>  
> - out:
>  	local_irq_restore(flags);
>  }
>