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Date: Thu, 9 Mar 2023 18:31:00 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Tero Kristo <tero.kristo@...ux.intel.com>
Cc: Daniel Bristot de Oliveira <bristot@...nel.org>,
mhiramat@...nel.org, linux-kernel@...r.kernel.org,
linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH] trace/hwlat: Do not restart per-cpu threads if they are
already running
On Thu, 2 Mar 2023 14:02:37 +0200
Tero Kristo <tero.kristo@...ux.intel.com> wrote:
> 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
I did the above and saw this:
# ps aux | grep hwlat
root 1410 36.2 0.0 0 0 ? R 18:10 0:22 [hwlatd/0]
root 1411 36.7 0.0 0 0 ? R 18:10 0:22 [hwlatd/1]
root 1412 36.4 0.0 0 0 ? R 18:10 0:22 [hwlatd/2]
root 1413 36.7 0.0 0 0 ? R 18:10 0:22 [hwlatd/3]
root 1414 36.7 0.0 0 0 ? R 18:10 0:22 [hwlatd/4]
root 1415 36.5 0.0 0 0 ? R 18:10 0:22 [hwlatd/5]
root 1417 36.4 0.0 0 0 ? R 18:10 0:22 [hwlatd/6]
root 1418 36.6 0.0 0 0 ? S 18:10 0:22 [hwlatd/7]
root 1426 33.1 0.0 0 0 ? R 18:10 0:11 [hwlatd/0]
root 1427 33.1 0.0 0 0 ? R 18:10 0:11 [hwlatd/1]
root 1428 32.9 0.0 0 0 ? R 18:10 0:11 [hwlatd/2]
root 1429 32.9 0.0 0 0 ? R 18:10 0:11 [hwlatd/3]
root 1430 32.8 0.0 0 0 ? R 18:10 0:11 [hwlatd/4]
root 1431 33.2 0.0 0 0 ? R 18:10 0:11 [hwlatd/5]
root 1432 33.2 0.0 0 0 ? R 18:10 0:11 [hwlatd/6]
root 1433 33.2 0.0 0 0 ? S 18:10 0:11 [hwlatd/7]
root 1521 0.0 0.0 6332 2048 pts/0 S+ 18:22 0:00 grep hwlat
Which I'm guessing is bad (having two of each hwlatd/#).
>
> 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.
>
After applying the patch and running the same commands, I have this:
# ps aux | grep hwlat
root 768 40.7 0.0 0 0 ? S 18:23 0:10 [hwlatd/0]
root 769 39.7 0.0 0 0 ? R 18:23 0:10 [hwlatd/1]
root 770 39.8 0.0 0 0 ? R 18:23 0:10 [hwlatd/2]
root 771 39.4 0.0 0 0 ? R 18:23 0:10 [hwlatd/3]
root 772 40.5 0.0 0 0 ? S 18:23 0:10 [hwlatd/4]
root 773 40.5 0.0 0 0 ? R 18:23 0:10 [hwlatd/5]
root 775 41.0 0.0 0 0 ? S 18:23 0:10 [hwlatd/6]
root 776 41.3 0.0 0 0 ? R 18:23 0:10 [hwlatd/7]
root 781 0.0 0.0 6332 2048 pts/0 S+ 18:25 0:00 grep hwlat
Looks as if this does fix the issue.
> >> 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;
> >> -
I believe this is two different bugs (the two hunks). This should be two
patches.
Please add the reproducer above to the commit log in a v2.
Thanks,
-- Steve
> >> for_each_cpu(cpu, current_mask) {
> >> retval = start_cpu_kthread(cpu);
> >> if (retval)
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