Messages in this thread Patch in this message | | | Date | Thu, 2 Mar 2023 08:49:53 -0300 | Subject | Re: [PATCH] trace/hwlat: Do not restart per-cpu threads if they are already running | From | Daniel Bristot de Oliveira <> |
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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.
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)
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