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

[Breno Leitao <leitao@debian.org>]

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@debian.org>
> Reported-by: Puranjay Mohan <puranjay@kernel.org>
> Reported-by: Usama Arif <usama.arif@linux.dev>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Reviewed-by: Breno Leitao <leitao@debian.org>

> 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);

Thanks for implementing it,
--breno