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 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.

- Josh Triplett