linux-kernel - Re: [PATCH] tracing/hwlat: Replace sched_setaffinity with set_cpus_allowed

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Message-ID: <51e64093-5e2f-18fa-db04-8d6789362921@kernel.org>
Date:   Thu, 16 Mar 2023 18:15:48 +0100
From:   Daniel Bristot de Oliveira <bristot@...nel.org>
To:     Costa Shulyupin <costa.shul@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        "open list:TRACING" <linux-kernel@...r.kernel.org>,
        "open list:TRACING" <linux-trace-kernel@...r.kernel.org>
Cc:     bwensley@...hat.com, constantine.shulyupin@...il.com
Subject: Re: [PATCH] tracing/hwlat: Replace sched_setaffinity with
 set_cpus_allowed_ptr

On 3/16/23 15:45, Costa Shulyupin wrote:
> There is a problem with the behavior of hwlat in a container,
> resulting in incorrect output. A warning message is generated:
> "cpumask changed while in round-robin mode, switching to mode none",
> and the tracing_cpumask is ignored. This issue arises because
> the kernel thread, hwlatd, is not a part of the container, and
> the function sched_setaffinity is unable to locate it using its PID.
> Additionally, the task_struct of hwlatd is already known.
> Ultimately, the function set_cpus_allowed_ptr achieves
> the same outcome as sched_setaffinity, but employs task_struct
> instead of PID.
> 
> Test case:
> 
>   # cd /sys/kernel/tracing
>   # echo 0 > tracing_on
>   # echo round-robin > hwlat_detector/mode
>   # echo hwlat > current_tracer
>   # unshare --fork --pid bash -c 'echo 1 > tracing_on'
>   # dmesg -c
> 
> Actual behavior:
> 
> [573502.809060] hwlat_detector: cpumask changed while in round-robin mode, switching to mode none
> 
> Signed-off-by: Costa Shulyupin <costa.shul@...hat.com>

I tested it and... it works.

Acked-by: Daniel Bristot de Oliveira <bristot@...nel.org>

Thanks
-- Daniel

> ---
>  kernel/trace/trace_hwlat.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/trace_hwlat.c b/kernel/trace/trace_hwlat.c
> index d440ddd5fd8b..444dfc31f258 100644
> --- a/kernel/trace/trace_hwlat.c
> +++ b/kernel/trace/trace_hwlat.c
> @@ -339,7 +339,7 @@ static void move_to_next_cpu(void)
>  	cpumask_clear(current_mask);
>  	cpumask_set_cpu(next_cpu, current_mask);
>  
> -	sched_setaffinity(0, current_mask);
> +	set_cpus_allowed_ptr(current, current_mask);
>  	return;
>  
>   change_mode:
> @@ -446,7 +446,7 @@ static int start_single_kthread(struct trace_array *tr)
>  
>  	}
>  
> -	sched_setaffinity(kthread->pid, current_mask);
> +	set_cpus_allowed_ptr(kthread, current_mask);
>  
>  	kdata->kthread = kthread;
>  	wake_up_process(kthread);