Re: [PATCH v3 1/3] rcutorture: Allow a negative value for nfakewriters

[Joel Fernandes <joelagnelf@nvidia.com>]

On Tue, Feb 25, 2025 at 12:00:18PM +0100, Uladzislau Rezki (Sony) wrote:
> Currently "nfakewriters" parameter can be set to any value but
> there is no possibility to adjust it automatically based on how
> many CPUs a system has where a test is run on.
> 
> To address this, if the "nfakewriters" is set to negative it will
> be adjusted to num_online_cpus() during torture initialization.
> 
> Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> ---
>  kernel/rcu/rcutorture.c | 22 ++++++++++++++++------
>  1 file changed, 16 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index d98b3bd6d91f..f376262532ce 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -148,6 +148,7 @@ MODULE_PARM_DESC(torture_type, "Type of RCU to torture (rcu, srcu, ...)");

IMO, this should also be updated to reflect the possibily to set it negative
and hence to number CPUs:

torture_param(int, nfakewriters, 4, "Number of RCU fake writer threads");

thanks,

 - Joel


>  
>  static int nrealnocbers;
>  static int nrealreaders;
> +static int nrealfakewriters;
>  static struct task_struct *writer_task;
>  static struct task_struct **fakewriter_tasks;
>  static struct task_struct **reader_tasks;
> @@ -1763,7 +1764,7 @@ rcu_torture_fakewriter(void *arg)
>  	do {
>  		torture_hrtimeout_jiffies(torture_random(&rand) % 10, &rand);
>  		if (cur_ops->cb_barrier != NULL &&
> -		    torture_random(&rand) % (nfakewriters * 8) == 0) {
> +		    torture_random(&rand) % (nrealfakewriters * 8) == 0) {
>  			cur_ops->cb_barrier();
>  		} else {
>  			switch (synctype[torture_random(&rand) % nsynctypes]) {
> @@ -2568,7 +2569,7 @@ rcu_torture_print_module_parms(struct rcu_torture_ops *cur_ops, const char *tag)
>  		 "nocbs_nthreads=%d nocbs_toggle=%d "
>  		 "test_nmis=%d "
>  		 "preempt_duration=%d preempt_interval=%d\n",
> -		 torture_type, tag, nrealreaders, nfakewriters,
> +		 torture_type, tag, nrealreaders, nrealfakewriters,
>  		 stat_interval, verbose, test_no_idle_hz, shuffle_interval,
>  		 stutter, irqreader, fqs_duration, fqs_holdoff, fqs_stutter,
>  		 test_boost, cur_ops->can_boost,
> @@ -3644,7 +3645,7 @@ rcu_torture_cleanup(void)
>  	rcu_torture_reader_mbchk = NULL;
>  
>  	if (fakewriter_tasks) {
> -		for (i = 0; i < nfakewriters; i++)
> +		for (i = 0; i < nrealfakewriters; i++)
>  			torture_stop_kthread(rcu_torture_fakewriter,
>  					     fakewriter_tasks[i]);
>  		kfree(fakewriter_tasks);
> @@ -4066,6 +4067,14 @@ rcu_torture_init(void)
>  
>  	rcu_torture_init_srcu_lockdep();
>  
> +	if (nfakewriters >= 0) {
> +		nrealfakewriters = nfakewriters;
> +	} else {
> +		nrealfakewriters = num_online_cpus() - 2 - nfakewriters;
> +		if (nrealfakewriters <= 0)
> +			nrealfakewriters = 1;
> +	}
> +
>  	if (nreaders >= 0) {
>  		nrealreaders = nreaders;
>  	} else {
> @@ -4122,8 +4131,9 @@ rcu_torture_init(void)
>  					  writer_task);
>  	if (torture_init_error(firsterr))
>  		goto unwind;
> -	if (nfakewriters > 0) {
> -		fakewriter_tasks = kcalloc(nfakewriters,
> +
> +	if (nrealfakewriters > 0) {
> +		fakewriter_tasks = kcalloc(nrealfakewriters,
>  					   sizeof(fakewriter_tasks[0]),
>  					   GFP_KERNEL);
>  		if (fakewriter_tasks == NULL) {
> @@ -4132,7 +4142,7 @@ rcu_torture_init(void)
>  			goto unwind;
>  		}
>  	}
> -	for (i = 0; i < nfakewriters; i++) {
> +	for (i = 0; i < nrealfakewriters; i++) {
>  		firsterr = torture_create_kthread(rcu_torture_fakewriter,
>  						  NULL, fakewriter_tasks[i]);
>  		if (torture_init_error(firsterr))
> -- 
> 2.39.5
>