Re: [RFC v4 3/4] irqflags: Avoid unnecessary calls to trace_ if you can

[Paul E. McKenney]

On Tue, Apr 24, 2018 at 10:26:58AM -0700, Paul E. McKenney wrote:
> On Tue, Apr 24, 2018 at 09:01:34AM -0700, Joel Fernandes wrote:
> > On Tue, Apr 24, 2018 at 8:56 AM, Paul E. McKenney
> > <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> > > On Mon, Apr 23, 2018 at 05:22:44PM -0400, Steven Rostedt wrote:
> > >> On Mon, 23 Apr 2018 13:12:21 -0400 (EDT)
> > >> Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> > >>
> > >>
> > >> > I'm inclined to explicitly declare the tracepoints with their given
> > >> > synchronization method. Tracepoint probe callback functions for currently
> > >> > existing tracepoints expect to have preemption disabled when invoked.
> > >> > This assumption will not be true anymore for srcu-tracepoints.
> > >>
> > >> Actually, why not have a flag attached to the tracepoint_func that
> > >> states if it expects preemption to be enabled or not? If a
> > >> trace_##event##_srcu() is called, then simply disable preemption before
> > >> calling the callbacks for it. That way if a callback is fine for use
> > >> with srcu, then it would require calling
> > >>
> > >>       register_trace_##event##_may_sleep();
> > >>
> > >> Then if someone uses this on a tracepoint where preemption is disabled,
> > >> we simply do not call it.
> > >
> > > One more stupid question...  If we are having to trace so much stuff
> > > in the idle loop, are we perhaps grossly overstating the extent of that
> > > "idle" loop?  For being called "idle", this code seems quite busy!
> > 
> > ;-)
> > The performance hit I am observing is when running a heavy workload,
> > like hackbench or something like that. That's what I am trying to
> > correct.
> > By the way is there any limitation on using SRCU too early during
> > boot? I backported Mathieu's srcu tracepoint patches but the kernel
> > hangs pretty early in the boot. I register lockdep probes in
> > start_kernel. I am hoping that's not why.
> > 
> > I could also have just screwed up the backporting... may be for my
> > testing, I will just replace the rcu API with the srcu instead of all
> > of Mathieu's new TRACE_EVENT macros for SRCU, since all I am trying to
> > do right now is measure the performance of my patches with SRCU.
> 
> Gah, yes, there is an entry on my capacious todo list on making SRCU
> grace periods work during early boot and mid-boot.  Let me see what
> I can do...

OK, just need to verify that you are OK with call_srcu()'s callbacks
not being invoked until sometime during core_initcall() time.  (If you
really do need them to be invoked before that, in theory it is possible,
but in practice it is weird, even for RCU.)

							Thanx, Paul