linux-kernel - Re: [RFC rcu/next] torture: Stop onoff task if there is only one cpu

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Date:	Mon, 2 May 2016 10:18:49 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	Boqun Feng <boqun.feng@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC rcu/next] torture: Stop onoff task if there is only one cpu

On Mon, May 02, 2016 at 10:30:00AM +0800, Boqun Feng wrote:
> If the whole system has only one cpu, that cpu won't be able to be
> offlined, so there is no need onoff task is stil running.
> 
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> ---
> 
> I hit something like the following while I was running rcutorture
> in a guest with only one vCPU:
> 
> [   31.197457] rcu-torture:torture_onoff task: offlining 0
> [   31.197508] rcu-torture:torture_onoff task: offline 0 failed: errno -16
> 
> I know this is an expected behavior, but think we could just stop
> the onoff task if there is only one cpu.

I find it a little bit unfortunate that this kicks off a thread just to
immediately exit that thread, rather than never starting it in the first
place.  However, it also seems like the most convenient solution here,
and I don't see much point in going out of the way to optimize this test
for uniprocessor systems.

Reviewed-by: Josh Triplett <josh@...htriplett.org>

>  kernel/torture.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/kernel/torture.c b/kernel/torture.c
> index fb39a06bbef5..a85b7d61d9dd 100644
> --- a/kernel/torture.c
> +++ b/kernel/torture.c
> @@ -194,6 +194,12 @@ torture_onoff(void *arg)
>  	for_each_online_cpu(cpu)
>  		maxcpu = cpu;
>  	WARN_ON(maxcpu < 0);
> +
> +	if (maxcpu == 0) {
> +		VERBOSE_TOROUT_STRING("only one cpu is found, onoff is impossible");
> +		goto stop;
> +	}
> +
>  	if (onoff_holdoff > 0) {
>  		VERBOSE_TOROUT_STRING("torture_onoff begin holdoff");
>  		schedule_timeout_interruptible(onoff_holdoff);
> @@ -209,6 +215,8 @@ torture_onoff(void *arg)
>  				       &sum_online, &min_online, &max_online);
>  		schedule_timeout_interruptible(onoff_interval);
>  	}
> +
> +stop:
>  	torture_kthread_stopping("torture_onoff");
>  	return 0;
>  }
> -- 
> 2.8.0
>