Re: [PATCH] perf ftrace: Factor regs retrieval for function tracer

[Peter Zijlstra]

On Thu, Dec 18, 2014 at 03:08:47PM +0100, Jiri Olsa wrote:
> diff --git a/kernel/trace/trace_event_perf.c b/kernel/trace/trace_event_perf.c
> index 6fa484de2ba1..54bffedd28b6 100644
> --- a/kernel/trace/trace_event_perf.c
> +++ b/kernel/trace/trace_event_perf.c
> @@ -304,7 +304,7 @@ perf_ftrace_function_call(unsigned long ip, unsigned long 
> parent_ip,
>  {
>       struct ftrace_entry *entry;
>       struct hlist_head *head;
> -     struct pt_regs regs;
> +     struct pt_regs **regsp, *regs = pt_regs;
>       int rctx;
>  
>       head = this_cpu_ptr(event_function.perf_events);
> @@ -316,16 +316,24 @@ perf_ftrace_function_call(unsigned long ip, unsigned 
> long parent_ip,
>  
>       BUILD_BUG_ON(ENTRY_SIZE > PERF_MAX_TRACE_SIZE);
>  
> -     perf_fetch_caller_regs(&regs);
> +     /*
> +      * The ftrace_ops is registered with SAVE_REGS_IF_SUPPORTED,
> +      * so if we got pt_regs defined, we don't need to retrieve
> +      * regs buffer through perf_trace_buf_prepare.
> +      */
> +     regsp = pt_regs ? NULL : &regs;
>  
> -     entry = perf_trace_buf_prepare(ENTRY_SIZE, TRACE_FN, NULL, &rctx);
> +     entry = perf_trace_buf_prepare(ENTRY_SIZE, TRACE_FN, regsp, &rctx);
>       if (!entry)
>               return;
>  
> +     if (regsp)
> +             perf_fetch_caller_regs(regs);
> +


Would not something like:

        struct pt_regs *regs;

        entry = perf_trace_buf_prepare(ENTRY_SIZE, TRACE_FN, &regs, &rctx);
        if (!pt_regs) {
                perf_fetch_caller_regs(regs);
                pt_regs = regs;
        }

Be simpler?
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