Re: [PATCH v4 16/27] tracing: Remove regular RCU context for _rcuidle tracepoints (again)

Peter Zijlstra
2020-03-06T10:44:30Z

On Fri, Feb 21, 2020 at 02:34:32PM +0100, Peter Zijlstra wrote:
> Effectively revert commit 865e63b04e9b2 ("tracing: Add back in
> rcu_irq_enter/exit_irqson() for rcuidle tracepoints") now that we've
> taught perf how to deal with not having an RCU context provided.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> ---
> include/linux/tracepoint.h | 8 ++------
> 1 file changed, 2 insertions(+), 6 deletions(-)
>
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -179,10 +179,8 @@ static inline struct tracepoint *tracepo
> * For rcuidle callers, use srcu since sched-rcu \
> * doesn't work from the idle path. \
> */ \
> - if (rcuidle) { \
> + if (rcuidle) \
> __idx = srcu_read_lock_notrace(&tracepoint_srcu);\
> - rcu_irq_enter_irqsave(); \
> - } \
> \
> it_func_ptr = rcu_dereference_raw((tp)->funcs); \
> \
> @@ -194,10 +192,8 @@ static inline struct tracepoint *tracepo
> } while ((++it_func_ptr)->func); \
> } \
> \
> - if (rcuidle) { \
> - rcu_irq_exit_irqsave(); \
> + if (rcuidle) \
> srcu_read_unlock_notrace(&tracepoint_srcu, __idx);\
> - } \
> \
> preempt_enable_notrace(); \
> } while (0)

So what happens when BPF registers for these tracepoints? BPF very much
wants RCU on AFAIU.