From: Steven Rostedt <rostedt@goodmis.org>
To: Erica Bugden <erica.bugden@linutronix.de>
Cc: linux-kernel@vger.kernel.org, peterz@infradead.org,
tglx@linutronix.de, anna-maria@linutronix.de,
bigeasy@linutronix.de
Subject: Re: [PATCH] ftrace: Add missing check for existing hwlat thread
Date: Wed, 1 Aug 2018 15:40:06 -0400 [thread overview]
Message-ID: <20180801154006.580806cc@gandalf.local.home> (raw)
In-Reply-To: <1533120354-22923-1-git-send-email-erica.bugden@linutronix.de>
On Wed, 1 Aug 2018 12:45:54 +0200
Erica Bugden <erica.bugden@linutronix.de> wrote:
> The hwlat tracer uses a kernel thread to measure latencies. The function
> that creates this kernel thread, start_kthread(), can be called when the
> tracer is initialized and when the tracer is explicitly enabled.
> start_kthread() does not check if there is an existing hwlat kernel
> thread and will create a new one each time it is called.
>
> This causes the reference to the previous thread to be lost. Without the
> thread reference, the old kernel thread becomes unstoppable and
> continues to use CPU time even after the hwlat tracer has been disabled.
> This problem can be observed when a system is booted with tracing
> enabled and the hwlat tracer is configured like this:
>
> echo hwlat > current_tracer; echo 1 > tracing_on
>
> Add the missing check for an existing kernel thread in start_kthread()
> to prevent this problem. This function and the rest of the hwlat kernel
> thread setup and teardown are already serialized because they are called
> through the tracer core code with trace_type_lock held.
>
> Signed-off-by: Erica Bugden <erica.bugden@linutronix.de>
> ---
> kernel/trace/trace_hwlat.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/kernel/trace/trace_hwlat.c b/kernel/trace/trace_hwlat.c
> index d7c8e4e..2d9d36d 100644
> --- a/kernel/trace/trace_hwlat.c
> +++ b/kernel/trace/trace_hwlat.c
> @@ -354,6 +354,9 @@ static int start_kthread(struct trace_array *tr)
> struct task_struct *kthread;
> int next_cpu;
>
> + if (hwlat_kthread)
> + return 0;
> +
This looks like it is treating the symptom and not the disease.
> /* Just pick the first CPU on first iteration */
> current_mask = &save_cpumask;
> get_online_cpus();
Can you try this patch?
-- Steve
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 823687997b01..15862044db05 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -7628,7 +7628,9 @@ rb_simple_write(struct file *filp, const char __user *ubuf,
if (buffer) {
mutex_lock(&trace_types_lock);
- if (val) {
+ if (!!val == tracer_tracing_is_on(tr)) {
+ val = 0; /* do nothing */
+ } else if (val) {
tracer_tracing_on(tr);
if (tr->current_trace->start)
tr->current_trace->start(tr);
next prev parent reply other threads:[~2018-08-01 19:40 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-01 10:45 [PATCH] ftrace: Add missing check for existing hwlat thread Erica Bugden
2018-08-01 19:40 ` Steven Rostedt [this message]
2018-08-01 19:59 ` Thomas Gleixner
2018-08-01 20:03 ` Steven Rostedt
2018-08-01 20:07 ` Thomas Gleixner
2018-08-03 8:46 ` Erica Bugden
2018-08-03 13:37 ` Steven Rostedt
2018-08-03 14:57 ` Erica Bugden
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