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

Peter Zijlstra
2020-03-06T11:31:35Z

On Fri, Mar 06, 2020 at 11:43:35AM +0100, Peter Zijlstra wrote:
> 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.

I suspect we needs something like this...

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index a2f15222f205..67a39dbce0ce 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -1475,11 +1475,13 @@ void bpf_put_raw_tracepoint(struct bpf_raw_event_map *btp)
static __always_inline
void __bpf_trace_run(struct bpf_prog *prog, u64 *args)
{
+ int rcu_flags = trace_rcu_enter();
rcu_read_lock();
preempt_disable();
(void) BPF_PROG_RUN(prog, args);
preempt_enable();
rcu_read_unlock();
+ trace_rcu_exit(rcu_flags);
}

#define UNPACK(...) __VA_ARGS__