Re: [PATCH tip/core/rcu 6/6] rcu: Make rcutorture fakewriters invoke rcu_barrier()

[Josh Triplett <josh@joshtriplett.org>]

On Fri, Jun 15, 2012 at 02:19:02PM -0700, Paul E. McKenney wrote:
> On Fri, Jun 15, 2012 at 01:37:05PM -0700, Josh Triplett wrote:
> > On Fri, Jun 15, 2012 at 11:57:54AM -0700, Paul E. McKenney wrote:
> > > From: "Paul E. McKenney" <paul.mckenney@linaro.org>
> > > 
> > > The current rcutorture rcu_barrier() testing never intentionally runs
> > > more than one instance of rcu_barrier() at a given time.  This fails
> > > to test the the shiny new concurrency features of rcu_barrier().  This
> > > commit therefore modifies the rcutorture fakewriter kthread to randomly
> > > invoke rcu_barrier() rather than the usual synchronize_rcu().
> > > 
> > > Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > ---
> > >  kernel/rcutorture.c |    6 +++++-
> > >  1 files changed, 5 insertions(+), 1 deletions(-)
> > > 
> > > diff --git a/kernel/rcutorture.c b/kernel/rcutorture.c
> > > index 54a3745..dfb4e20 100644
> > > --- a/kernel/rcutorture.c
> > > +++ b/kernel/rcutorture.c
> > > @@ -1025,7 +1025,11 @@ rcu_torture_fakewriter(void *arg)
> > >  	do {
> > >  		schedule_timeout_uninterruptible(1 + rcu_random(&rand)%10);
> > >  		udelay(rcu_random(&rand) & 0x3ff);
> > > -		cur_ops->sync();
> > > +		if (cur_ops->cb_barrier != NULL &&
> > > +		    rcu_random(&rand) % (NR_CPUS * 8) == 0)
> > 
> > NR_CPUS seems like an odd choice here.  I assume you want to control for
> > having many rcu_torture_fakewriter threads, and aim for the same average
> > rate of barrier calls across the whole set of threads regardless of the
> > number of threads.  However, NR_CPUS does not accurately reflect either
> > the number of fakewriter threads (which a user can set arbitrarily) or
> > the number of CPUs currently on the system (since NR_CPUS represents the
> > compile-time limit).  I'd suggest changing this to use the actual number
> > of fakewriter threads, which rcutorture knows at start time.
> 
> Indeed, this should use the number of online CPUs.  Which should be
> easy to compute, will fix.

I'd suggest using nfakewriters instead.

- Josh Triplett