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

[Mathieu Desnoyers]

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

> On Fri, Mar 6, 2020 at 3:31 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
>> > > Reviewed-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
>> > > ---
>> > >  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.
http://www.efficios.com