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Date:   Sun, 19 Mar 2023 12:46:47 -0400
From:   Steven Rostedt <>
Cc:     Masami Hiramatsu <>,
        Mark Rutland <>,
        Andrew Morton <>,, Tero Kristo <>,
        Daniel Bristot de Oliveira <>
Subject: [for-linus][PATCH 4/8] trace/hwlat: Do not start per-cpu thread if it is already running

From: Tero Kristo <>

The hwlatd tracer will end up starting multiple per-cpu threads with
the following script:

    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 1 > tracing_on

To fix the issue, check if the hwlatd thread for the cpu is already
running, before starting a new one. Along with the previous patch, this
avoids running multiple instances of the same CPU thread on the system.


Fixes: f46b16520a087 ("trace/hwlat: Implement the per-cpu mode")
Signed-off-by: Tero Kristo <>
Acked-by: Daniel Bristot de Oliveira <>
Signed-off-by: Steven Rostedt (Google) <>
 kernel/trace/trace_hwlat.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/trace/trace_hwlat.c b/kernel/trace/trace_hwlat.c
index edc26dc22c3f..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");

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