RE: [PATCH v2] rcutorture: Create nocb tasks only for CONFIG_RCU_NOCB_CPU=y kernels

[Zhang, Qiang1]

> On Feb 2, 2023, at 1:57 AM, Zqiang <qiang1.zhang@xxxxxxxxx> wrote:
> 
> When setting nocbs_nthreads to start rcutorture test with a non-zero value,
> the nocb tasks will be created and invoke rcu_nocb_cpu_offload/deoffload()
> to toggle CPU's callback-offload state, but for CONFIG_RCU_NOCB_CPU=n
> kernel, the rcu_nocb_cpu_offload/deoffload() is a no-op and this is also
> meaningless for torture_type is non-rcu.
> 
> This commit therefore add member can_nocbs_toggle to rcu_torture_ops
> structure to avoid unnecessary nocb tasks creation.
> 
> Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
> ---
> kernel/rcu/rcutorture.c | 10 ++++++++--
> 1 file changed, 8 insertions(+), 2 deletions(-)
>
>Sorry if I am missing something but what is the point of adding more lines of code and complexity to handle this? Does it improve the test coverage or reduce overhead?
>
>This is test code. I see no problem with cost of an extra unused task with positive trade off of keeping the code simple…

For nocbs_nthreads is non-zero and CONFIG_RCU_NOCB_CPU=n kernels, 
the rcu_nocb_cpu_offload/deoffload() is a no-op,  we create nocbs_nthreads 
kthreads and perform nocb toggle tests periodically, which is meaningless and
will take extra cpu time.

For non-rcu tests, it really doesn't make sense for us to turn on nocb toggle test.

Does this make the test a little more rigorous?

Thanks
Zqiang

>
>thanks,
>
> - Joel 


> 
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index 297da28ce92d..ced0a8e1d765 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -383,6 +383,7 @@ struct rcu_torture_ops {
>    long cbflood_max;
>    int irq_capable;
>    int can_boost;
> +    int can_nocbs_toggle;
>    int extendables;
>    int slow_gps;
>    int no_pi_lock;
> @@ -569,6 +570,7 @@ static struct rcu_torture_ops rcu_ops = {
>    .stall_dur        = rcu_jiffies_till_stall_check,
>    .irq_capable        = 1,
>    .can_boost        = IS_ENABLED(CONFIG_RCU_BOOST),
> +    .can_nocbs_toggle    = IS_ENABLED(CONFIG_RCU_NOCB_CPU),
>    .extendables        = RCUTORTURE_MAX_EXTEND,
>    .name            = "rcu"
> };
> @@ -2356,7 +2358,7 @@ rcu_torture_print_module_parms(struct rcu_torture_ops *cur_ops, const char *tag)
>         "n_barrier_cbs=%d "
>         "onoff_interval=%d onoff_holdoff=%d "
>         "read_exit_delay=%d read_exit_burst=%d "
> -         "nocbs_nthreads=%d nocbs_toggle=%d "
> +         "nocbs_nthreads=%d/%d nocbs_toggle=%d "
>         "test_nmis=%d\n",
>         torture_type, tag, nrealreaders, nfakewriters,
>         stat_interval, verbose, test_no_idle_hz, shuffle_interval,
> @@ -2368,7 +2370,7 @@ rcu_torture_print_module_parms(struct rcu_torture_ops *cur_ops, const char *tag)
>         n_barrier_cbs,
>         onoff_interval, onoff_holdoff,
>         read_exit_delay, read_exit_burst,
> -         nocbs_nthreads, nocbs_toggle,
> +         nocbs_nthreads, cur_ops->can_nocbs_toggle, nocbs_toggle,
>         test_nmis);
> }
> 
> @@ -3708,6 +3710,10 @@ rcu_torture_init(void)
>        pr_alert("rcu-torture: ->fqs NULL and non-zero fqs_duration, fqs disabled.\n");
>        fqs_duration = 0;
>    }
> +    if (!cur_ops->can_nocbs_toggle && nocbs_nthreads != 0) {
> +        pr_alert("rcu-torture: ->can_nocbs_toggle false and non-zero nocbs_nthreads, nocbs_toggle disabled.\n");
> +        nocbs_nthreads = 0;
> +    }
>    if (cur_ops->init)
>        cur_ops->init();
> 
> -- 
> 2.25.1
>