Re: [PATCH RFC] trace: correct off by one preempt counter while recording the trace-event

[Steven Rostedt]

On Wed, 25 May 2016 15:25:37 +0200
Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> wrote:

> ---
> I am not sure if this is the only spot that needs correction of the
> preemption counter. If it is it could be corrected directly in
> trace_event_buffer_reserve().
> 
>  include/linux/tracepoint.h  | 13 +++++++++++++
>  kernel/trace/trace_events.c |  2 +-
>  2 files changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> index be586c632a0c..12cb3bb40c1c 100644
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -33,6 +33,19 @@ struct trace_enum_map {
>  
>  #define TRACEPOINT_DEFAULT_PRIO	10
>  
> +/*
> + * The preempt count recorded in trace_event_raw_event_# are off by one due to
> + * rcu_read_lock_sched_notrace() in __DO_TRACE. This is corrected here.
> + */
> +static inline int event_preempt_count(void)
> +{
> +#ifdef CONFIG_PREEMPT
> +	return preempt_count() - 1;
> +#else
> +	return 0;
> +#endif
> +}
> +
>  extern int
>  tracepoint_probe_register(struct tracepoint *tp, void *probe, void *data);
>  extern int
> diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> index 52c4fffaddcd..90b40cf6ec98 100644
> --- a/kernel/trace/trace_events.c
> +++ b/kernel/trace/trace_events.c
> @@ -245,7 +245,7 @@ void *trace_event_buffer_reserve(struct trace_event_buffer *fbuffer,
>  		return NULL;
>  
>  	local_save_flags(fbuffer->flags);
> -	fbuffer->pc = preempt_count();
> +	fbuffer->pc = event_preempt_count();
>  	fbuffer->trace_file = trace_file;
>  
>  	fbuffer->event =

Hmm, what about this? This is a bit cleaner IMO.

-- Steve