tracing/osnoise: Switch from PF_NO_SETAFFINITY to migrate_disable

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.

Link: https://lkml.kernel.org/r/8ba8bc9c15b3ea40cf73cf67a9bc061a264609f0.1686063934.git.bristot@kernel.org
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: William White <chwhite@redhat.com>
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>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
index e97e3fa5cbed0ae0ed36e60106194a60b1fa07ba..c265ec5f1726c5864ccc94ebd87d5691a50c856f 100644 (file)
--- 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 */