Re: [PATCH] trace/hwlat: Do not restart per-cpu threads if they are already running

[Daniel Bristot de Oliveira <bristot@kernel.org>]

Hi Tero,

On 3/2/23 08:36, Tero Kristo wrote:
> Check if the hwlatd thread for the cpu is already running, before
> starting a new one. This avoids running multiple instances of the same
> CPU thread on the system. Also, do not wipe the contents of the
> per-cpu kthread data when starting the tracer, as this can completely
> forget about already running instances and start new additional per-cpu
> threads. Fixes issues where fiddling with either the mode of the hwlat
> tracer or doing cpu-hotplugs messes up the internal book-keeping
> resulting in stale hwlatd threads.

Thanks for your patch.

Would you mind explaining how do you hit the problem? that is, how can
I reproduce the same problem you faced.

I tried reproducing it by dispatching the hwlat tracer in two instances,
but the system already blocks me...

[root@vm tracing]# echo hwlat > current_tracer 
[root@vm tracing]# cd instances/
[root@vm instances]# mkdir hwlat_2
[root@vm instances]# cd hwlat_2/
[root@vm hwlat_2]# echo hwlat > current_tracer 
-bash: echo: write error: Device or resource busy

[root@vm hwlat_2]# cd ../../
[root@vm tracing]# echo nop > current_tracer 
[root@vm tracing]# cd instances/hwlat_2/
[root@vm hwlat_2]# echo hwlat > current_tracer 
[root@vm hwlat_2]# cd ..
[root@vm instances]# mkdir hwlat_1
[root@vm instances]# cd hwlat_1/
[root@vm hwlat_1]# echo hwlat > current_tracer 
-bash: echo: write error: Device or resource busy
[root@vm hwlat_1]# 

Having a reproducer helps us to think better about the problem.

-- Daniel

> Signed-off-by: Tero Kristo <tero.kristo@linux.intel.com>
> ---
>  kernel/trace/trace_hwlat.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/trace/trace_hwlat.c b/kernel/trace/trace_hwlat.c
> index d440ddd5fd8b..c4945f8adc11 100644
> --- a/kernel/trace/trace_hwlat.c
> +++ b/kernel/trace/trace_hwlat.c
> @@ -492,6 +492,10 @@ static int start_cpu_kthread(unsigned int cpu)
>  {
>  	struct task_struct *kthread;
>  
> +	/* Do not start a new hwlatd thread if it is already running */
> +	if (per_cpu(hwlat_per_cpu_data, cpu).kthread)
> +		return 0;
> +
>  	kthread = kthread_run_on_cpu(kthread_fn, NULL, cpu, "hwlatd/%u");
>  	if (IS_ERR(kthread)) {
>  		pr_err(BANNER "could not start sampling thread\n");
> @@ -584,9 +588,6 @@ static int start_per_cpu_kthreads(struct trace_array *tr)
>  	 */
>  	cpumask_and(current_mask, cpu_online_mask, tr->tracing_cpumask);
>  
> -	for_each_online_cpu(cpu)
> -		per_cpu(hwlat_per_cpu_data, cpu).kthread = NULL;
> -
>  	for_each_cpu(cpu, current_mask) {
>  		retval = start_cpu_kthread(cpu);
>  		if (retval)