[PATCH V3 1/3] tracing/osnoise: Switch from PF_NO_SETAFFINITY to migrate_disable

Daniel Bristot de Oliveira
2023-06-06T15:12:48Z

Currently, osnoise/timerlat threads run with PF_NO_SETAFFINITY set.
It works well, however, cgroups do not allow PF_NO_SETAFFINITY threads
to be accepted, and this creates a limitation to osnoise/timerlat.

To avoid this limitation, disable migration of the threads as soon
as they start to run, and then clean the PF_NO_SETAFFINITY flag (still)
used during thread creation.

If for some reason a thread migration is requested, e.g., via
sched_settafinity, the tracer thread will notice and exit.

Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Daniel Bristot de Oliveira <bristot@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
---
kernel/trace/trace_osnoise.c | 68 ++++++++++++++++++++++++++++++++++++
1 file changed, 68 insertions(+)

diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
index e97e3fa5cbed..c265ec5f1726 100644
--- a/kernel/trace/trace_osnoise.c
+++ b/kernel/trace/trace_osnoise.c
@@ -1545,6 +1545,39 @@ static void osnoise_sleep(void)
}
}

+/*
+ * osnoise_migration_pending - checks if the task needs to migrate
+ *
+ * osnoise/timerlat threads are per-cpu. If there is a pending request to
+ * migrate the thread away from the current CPU, something bad has happened.
+ * Play the good citizen and leave.
+ *
+ * Returns 0 if it is safe to continue, 1 otherwise.
+ */
+static inline int osnoise_migration_pending(void)
+{
+ if (!current->migration_pending)
+ return 0;
+
+ /*
+ * If migration is pending, there is a task waiting for the
+ * tracer to enable migration. The tracer does not allow migration,
+ * thus: taint and leave to unblock the blocked thread.
+ */
+ osnoise_taint("migration requested to osnoise threads, leaving.");
+
+ /*
+ * Unset this thread from the threads managed by the interface.
+ * The tracers are responsible for cleaning their env before
+ * exiting.
+ */
+ mutex_lock(&interface_lock);
+ this_cpu_osn_var()->kthread = NULL;
+ mutex_unlock(&interface_lock);
+
+ return 1;
+}
+
/*
* osnoise_main - The osnoise detection kernel thread
*
@@ -1553,12 +1586,29 @@ static void osnoise_sleep(void)
*/
static int osnoise_main(void *data)
{
+ unsigned long flags;
+
+ /*
+ * This thread was created pinned to the CPU using PF_NO_SETAFFINITY.
+ * The problem is that cgroup does not allow PF_NO_SETAFFINITY thread.
+ *
+ * To work around this limitation, disable migration and remove the
+ * flag.
+ */
+ migrate_disable();
+ raw_spin_lock_irqsave(&current->pi_lock, flags);
+ current->flags &= ~(PF_NO_SETAFFINITY);
+ raw_spin_unlock_irqrestore(&current->pi_lock, flags);

while (!kthread_should_stop()) {
+ if (osnoise_migration_pending())
+ break;
+
run_osnoise();
osnoise_sleep();
}

+ migrate_enable();
return 0;
}

@@ -1706,6 +1756,7 @@ static int timerlat_main(void *data)
struct timerlat_variables *tlat = this_cpu_tmr_var();
struct timerlat_sample s;
struct sched_param sp;
+ unsigned long flags;
u64 now, diff;

/*
@@ -1714,6 +1765,18 @@ static int timerlat_main(void *data)
sp.sched_priority = DEFAULT_TIMERLAT_PRIO;
sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);

+ /*
+ * This thread was created pinned to the CPU using PF_NO_SETAFFINITY.
+ * The problem is that cgroup does not allow PF_NO_SETAFFINITY thread.
+ *
+ * To work around this limitation, disable migration and remove the
+ * flag.
+ */
+ migrate_disable();
+ raw_spin_lock_irqsave(&current->pi_lock, flags);
+ current->flags &= ~(PF_NO_SETAFFINITY);
+ raw_spin_unlock_irqrestore(&current->pi_lock, flags);
+
tlat->count = 0;
tlat->tracing_thread = false;

@@ -1731,6 +1794,7 @@ static int timerlat_main(void *data)
osn_var->sampling = 1;

while (!kthread_should_stop()) {
+
now = ktime_to_ns(hrtimer_cb_get_time(&tlat->timer));
diff = now - tlat->abs_period;

@@ -1749,10 +1813,14 @@ static int timerlat_main(void *data)
if (time_to_us(diff) >= osnoise_data.stop_tracing_total)
osnoise_stop_tracing();

+ if (osnoise_migration_pending())
+ break;
+
wait_next_period(tlat);
}

hrtimer_cancel(&tlat->timer);
+ migrate_enable();
return 0;
}
#else /* CONFIG_TIMERLAT_TRACER */
--
2.38.1