On Fri, Apr 07, 2017 at 10:58:26AM -0400, Steven Rostedt wrote:
> On Fri, 7 Apr 2017 07:43:35 -0700
> "Paul E. McKenney" <[email protected]> wrote:
>
> > On Fri, Apr 07, 2017 at 10:01:06AM -0400, Steven Rostedt wrote:
> > > Paul,
> > >
> > > Here's my latest. You OK with it?
> >
> > Given your update to 3/5, I suspect that we could live with it. I am
> > expecting some complaints about increases in idle-entry latency, but might
> > be best to wait for complaints rather than complexifying too proactively.
>
> We only added a this_cpu_inc() and this_cpu_dec() which are very fast
> operations. I highly doubt it will be measurable. Although, I'm talking
> about x86, IIRC, the this_cpu_inc/dec were be poorly written for other
> archs in the past. I'm not sure if that was fixed though.
That is an issue for CPUs that don't have a to-memory increment
instruction. How about __this_cpu_inc() and __this_cpu_dec(), given
that preemption is disabled?
> > That said, there isn't supposed to be any tracing during the now very
> > small interval where RCU's idle-entry is incomplete. Mightn't it be
> > better to (under CONFIG_PROVE_RCU or some such) give splats if tracing
> > showed up in that interval?
>
> Again, tracing is not the issue. I do function tracing in that location
> without any problems. The issue here was the stack tracer.
>
> Maybe we can create a new variable that is more cache local to the RCU
> code.
>
> What about calling it "rcu_disabled"? Then tracing that depends on RCU
> can simply check that.
>
> s/stack_trace_disable/disable_rcu/
> s/stack_trace_enable/enable_rcu/
>
> export a per cpu variable rcu_disabled
>
> Then I can have the stack tracer check that variable as well. And we
> could even put in a WARN_ON(this_cpu_read(rcu_disabled) in the
> TRACE_EVENT() macros.
>
> Thoughts?
At this point, if you can use the "__" versions, the change should be
small. With that change, if no one else complains, I am OK.
Thanx, Paul
