linux-kernel - Re: [PATCH] rcutorture: Traverse possible cpu to set maxcpu in rcu_nocb

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Date:   Thu, 24 Aug 2023 06:21:03 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Zqiang <qiang.zhang1211@...il.com>
Cc:     joel@...lfernandes.org, linux-kernel@...r.kernel.org,
        rcu@...r.kernel.org
Subject: Re: [PATCH] rcutorture: Traverse possible cpu to set maxcpu in
 rcu_nocb_toggle()

On Thu, Aug 24, 2023 at 04:42:06PM +0800, Zqiang wrote:
> Currently, the maxcpu is set by traversing online CPUs, however, if
> the rcutorture.onoff_holdoff is set zero and onoff_interval is set
> non-zero, and the some CPUs with larger cpuid has been offline before
> setting maxcpu, for these CPUs, even if they are online again, also
> cannot be offload or deoffload.
> 
> This commit therefore use for_each_possible_cpu() instead of
> for_each_online_cpu() in rcu_nocb_toggle().
> 
> Signed-off-by: Zqiang <qiang.zhang1211@...il.com>
> ---
>  kernel/rcu/rcutorture.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index a58372bdf0c1..b75d0fe558ce 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -2131,7 +2131,7 @@ static int rcu_nocb_toggle(void *arg)
>  	VERBOSE_TOROUT_STRING("rcu_nocb_toggle task started");
>  	while (!rcu_inkernel_boot_has_ended())
>  		schedule_timeout_interruptible(HZ / 10);
> -	for_each_online_cpu(cpu)
> +	for_each_possible_cpu(cpu)

Last I checked, bad things could happen if the code attempted to
nocb_toggle a CPU that had not yet come online.  Has that changed?

							Thanx, Paul

>  		maxcpu = cpu;
>  	WARN_ON(maxcpu < 0);
>  	if (toggle_interval > ULONG_MAX)
> -- 
> 2.17.1
>