linux-kernel - Re: [PATCH tip/core/rcu 6/6] rcu: Make rcutorture fakewriters invoke rcu

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Message-ID: <20120615203705.GE31184@leaf>
Date:	Fri, 15 Jun 2012 13:37:05 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, patches@...aro.org,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH tip/core/rcu 6/6] rcu: Make rcutorture fakewriters invoke
 rcu_barrier()

On Fri, Jun 15, 2012 at 11:57:54AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul.mckenney@...aro.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@...aro.org>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.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
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