Messages in this thread Patch in this message | | | Date | Thu, 2 Mar 2023 14:02:37 +0200 | Subject | Re: [PATCH] trace/hwlat: Do not restart per-cpu threads if they are already running | From | Tero Kristo <> |
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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
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.
-Tero
> > 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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