Re: RCU_BOOST not working for me

[Joel Fernandes <joel@joelfernandes.org>]

On Fri, Jan 17, 2020 at 08:34:58PM -0800, Paul E. McKenney wrote:
> On Fri, Jan 17, 2020 at 09:34:34PM -0500, Joel Fernandes wrote:
> > On Fri, Jan 17, 2020 at 03:17:56PM -0800, Paul E. McKenney wrote:
> > [...] 
> > > But rcutorture already has tests for RCU priority boosting.  Or are those
> > > failing in some way?
> > 
> > Yes there are tests, but I thought of just a simple experiment to study this.
> > Purely since it is existing RCU kernel code that I'd like to understand. And
> > me/Daniel are also looking into possibly using run-time / trace-based
> > verification some of these behaviors.
> 
> The functionality of rcu_state.cbovld should make that more entertaining.
> 
> But I would guess that the initial model would ignore memory footprint
> and just model RCU priority boosting as kicking in a fixed time after
> the beginning of the grace period.
> 
> Or do you guys have something else in mind?

Yes, that is the idea. And then turn the model into a unit test (for the
measurement). Though I am also personally trying to convince myself that a
unit test based on a model is better than the test in the kernel module I
just posted. We're just looking at applying Daniel's modeling work to
verification of behaviors like these.

A poor-man's alternative of a model-based test is just making sure that
synchronize_rcu() finishes in a bounded period of time (basically test by
observation than test by model) similar to what my kernel module did.  But I
guess a model based test would be more accurate and more strict about what is
considered a pass vs fail.

I was also studying SRCU and could not find tracepoints so I am thinking of
adding some to aid the study. I know for Tree-SRCU you are using timers and
workqueues but the concept hasn't largely changed since [1] was written
right?

[1] https://lwn.net/Articles/202847/

thanks!

 - Joel

> 							Thanx, Paul
> 
> PS.  Steve, yes, I do well remember our earlier discussions about readers
>      inheriting priority from the highest-priority synchronize_rcu().  ;-)