linux-kernel - Re: [PATCH] tracing: Quiet smp_processor_id() use in preemptable warning in hwlat

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Date:   Thu, 5 Aug 2021 08:24:40 +0200
From:   Daniel Bristot de Oliveira <bristot@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] tracing: Quiet smp_processor_id() use in preemptable
 warning in hwlat

On 8/4/21 8:18 PM, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> 
> The hardware latency detector (hwlat) has a mode that it runs one thread
> across CPUs. The logic to move from the currently running CPU to the next
> one in the list does a smp_processor_id() to find where it currently is.
> Unfortunately, it's done with preemption enabled, and this triggers a
> warning for using smp_processor_id() in a preempt enabled section.
> 
> As it is only using smp_processor_id() to get information on where it
> currently is in order to simply move it to the next CPU, it doesn't really
> care if it got moved in the mean time. It will simply balance out later if
> such a case arises.
> 
> Switch smp_processor_id() to raw_smp_processor_id() to quiet that warning.
> 
> Fixes: 8fa826b7344d ("trace/hwlat: Implement the mode config option")
> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

Acked-by: Daniel Bristot de Oliveira <bristot@...hat.com>

Thanks!
-- Daniel

> ---
>  kernel/trace/trace_hwlat.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/trace_hwlat.c b/kernel/trace/trace_hwlat.c
> index a6c0cdaf4b87..14f46aae1981 100644
> --- a/kernel/trace/trace_hwlat.c
> +++ b/kernel/trace/trace_hwlat.c
> @@ -327,7 +327,7 @@ static void move_to_next_cpu(void)
>  
>  	get_online_cpus();
>  	cpumask_and(current_mask, cpu_online_mask, tr->tracing_cpumask);
> -	next_cpu = cpumask_next(smp_processor_id(), current_mask);
> +	next_cpu = cpumask_next(raw_smp_processor_id(), current_mask);
>  	put_online_cpus();
>  
>  	if (next_cpu >= nr_cpu_ids)
>