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

Mathieu Desnoyers
2020-03-06T16:04:30Z

----- On Mar 6, 2020, at 10:51 AM, Alexei Starovoitov alexei.starovoitov@gmail.com wrote:

> On Fri, Mar 6, 2020 at 3:31 AM Peter Zijlstra <peterz@infradead.org> wrote:
>>
>> 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);
>
> One big NACK.
> I will not slowdown 99% of cases because of one dumb user.
> Absolutely no way.

If we care about not adding those extra branches on the fast-path, there is
an alternative way to do things: BPF could provide two distinct probe callbacks,
one meant for rcuidle tracepoints (which would have the trace_rcu_enter/exit), and
the other for the for 99% of the other callsites which have RCU watching.

I would recommend performing benchmarks justifying the choice of one approach over
the other though.

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.