Re: [PATCH v2 06/13] rcutorture: Add nwriters module parameter

[Paul E. McKenney]

On Fri, Jul 31, 2026 at 01:22:19AM -0700, Breno Leitao wrote:
> On Thu, Jul 30, 2026 at 06:07:12PM -0700, Paul E. McKenney wrote:
> > Believe it or not, there are people who would like to run rcutorture
> > without actually torturing RCU.  For example, some people would like to
> > induce various types of stall warnings without placing any unnecessary
> > additional overhead on their systems running in production.  And
> > rcutorture provides the stall_cpu, stall_cpu_holdoff, stall_no_softlockup,
> > stall_cpu_irqsoff, stall_cpu_block, and stall_cpu_repeat module parameters
> > in order to allow the user to force numerous types of stalls.  In addition,
> > rcutorture provides a great number of other module parameters to allow the
> > user to reduce other overhead.
> > 
> > But unfortunately, there is no way to turn of the rcu_torture_writer()
> > portion of this torture test, which on my x86 laptop consumes somewhere
> > between 40% and 45% of a CPU.  Although this is quite lightweight for a
> > torture test, it is not welcome on systems running production workloads.
> > This commit therefore adds an nwriters module parameter that defaults
> > to 1 but can be set to 0 in order to disable the rcu_torture_writer()
> > portion of the torture test, but that cannot be set to any other value
> > (that is what the fakewriters module parameter is for!).  This reduces
> > the overhead to well under 1% of a CPU, which is much more likely to be
> > compatible with production workloads.
> > 
> > Reported-by: Breno Leitao <leitao@xxxxxxxxxx>
> > Reported-by: Puranjay Mohan <puranjay@xxxxxxxxxx>
> > Reported-by: Usama Arif <usama.arif@xxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> 
> Reviewed-by: Breno Leitao <leitao@xxxxxxxxxx>
> 
> > diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> ...
> > +torture_param(bool, nwriters, 1, "Number of RCU writer threads (0 or 1)");
> 
> It looks like you used `int` for the same parameter in hazptrtorure.c
> and bool here
> 
> 	torture_param(int, nwriters, 1, "Number of hazard-pointer writer threads, 0 or 1");
> 
> Needeless to say that bool is more appropriate, and you don't need to
> check it later.
> 
> 	WARN_ON(IS_BUILTIN(CONFIG_HAZPTR_TORTURE_TEST) && nwriters != 1);

Fair point!  Although the check is because I didn't think that it made
sense to do a built-in test without any writers.  Which sounds a bit
silly now.

I will change nwriters to "bool" and remove that WARN_ON(), thank you!

							Thanx, Paul