Patch "trace/hwlat: Do not start per-cpu thread if it is already running" has been added to the 5.15-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    trace/hwlat: Do not start per-cpu thread if it is already running

to the 5.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     trace-hwlat-do-not-start-per-cpu-thread-if-it-is-alr.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 2044e259d126889af184d670b374ca8d6ae56c40
Author: Tero Kristo <tero.kristo@xxxxxxxxxxxxxxx>
Date:   Fri Mar 10 12:04:51 2023 +0200

    trace/hwlat: Do not start per-cpu thread if it is already running
    
    [ Upstream commit 08697bca9bbba15f2058fdbd9f970bd5f6a8a2e8 ]
    
    The hwlatd tracer will end up starting multiple per-cpu threads with
    the following script:
    
        #!/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 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.
    
    Link: https://lore.kernel.org/all/20230302113654.2984709-1-tero.kristo@xxxxxxxxxxxxxxx/
    Link: https://lkml.kernel.org/r/20230310100451.3948583-3-tero.kristo@xxxxxxxxxxxxxxx
    
    Cc: stable@xxxxxxxxxxxxxxx
    Fixes: f46b16520a087 ("trace/hwlat: Implement the per-cpu mode")
    Signed-off-by: Tero Kristo <tero.kristo@xxxxxxxxxxxxxxx>
    Acked-by: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
    Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/kernel/trace/trace_hwlat.c b/kernel/trace/trace_hwlat.c
index 72eeab938f1de..9ec032f22531c 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");