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

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Date:	Fri, 15 Jun 2012 14:52:48 -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 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@...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.
> 
> 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
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