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

[Tero Kristo <tero.kristo@linux.intel.com>]

Hi,

On 02/03/2023 13:49, Daniel Bristot de Oliveira wrote:
> 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.

For example, this script snippet reproduces it for me every time:

#!/bin/sh
cd /sys/kernel/debug/tracing
echo 0 > tracing_on
echo hwlat > current_tracer
echo per-cpu > hwlat_detector/mode
echo 100000 > hwlat_detector/width
echo 200000 > hwlat_detector/window
echo 200 > tracing_thresh
echo 1 > tracing_on

Another case where something wonky happens is if you offline/online a 
large number of CPUs (which takes a lot of time), and you start/disable 
the hwlat tracer at the same time.

-Tero


>
> 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)