tracing: Have hwlat trace migrate across tracing_cpumask CPUs

Instead of having the hwlat detector thread stay on one CPU, have it migrate
across all the CPUs specified by tracing_cpumask. If the user modifies the
thread's CPU affinity, the migration will stop until the next instance that
the tracer is instantiated. The migration happens at the end of each window
(period).

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/Documentation/trace/hwlat_detector.txt b/Documentation/trace/hwlat_detector.txt
index c02e8ef800cf514a5d3c1dbfdbbf8b8023e60f19..3207717a0d1ac4c6e251f091e1f7d2dda3056aef 100644 (file)
--- a/Documentation/trace/hwlat_detector.txt
+++ b/Documentation/trace/hwlat_detector.txt
@@ -69,5 +69,11 @@ in /sys/kernel/tracing:
 
  tracing_threshold     - minimum latency value to be considered (usecs)
  tracing_max_latency   - maximum hardware latency actually observed (usecs)
+ tracing_cpumask       - the CPUs to move the hwlat thread across
  hwlat_detector/width  - specified amount of time to spin within window (usecs)
  hwlat_detector/window - amount of time between (width) runs (usecs)
+
+The hwlat detector's kernel thread will migrate across each CPU specified in
+tracing_cpumask between each window. To limit the migration, either modify
+tracing_cpumask, or modify the hwlat kernel thread (named [hwlatd]) CPU
+affinity directly, and the migration will stop.

diff --git a/kernel/trace/trace_hwlat.c b/kernel/trace/trace_hwlat.c
index 08dfabe4e862a1ee91ceee917b91a03aef707837..65aab3914a561be97fa15f8d7d92c59c472957ca 100644 (file)
--- a/kernel/trace/trace_hwlat.c
+++ b/kernel/trace/trace_hwlat.c
@@ -42,6 +42,7 @@
 #include <linux/kthread.h>
 #include <linux/tracefs.h>
 #include <linux/uaccess.h>
+#include <linux/cpumask.h>
 #include <linux/delay.h>
 #include "trace.h"
 
@@ -211,6 +212,57 @@ out:
        return ret;
 }
 
+static struct cpumask save_cpumask;
+static bool disable_migrate;
+
+static void move_to_next_cpu(void)
+{
+       static struct cpumask *current_mask;
+       int next_cpu;
+
+       if (disable_migrate)
+               return;
+
+       /* Just pick the first CPU on first iteration */
+       if (!current_mask) {
+               current_mask = &save_cpumask;
+               get_online_cpus();
+               cpumask_and(current_mask, cpu_online_mask, tracing_buffer_mask);
+               put_online_cpus();
+               next_cpu = cpumask_first(current_mask);
+               goto set_affinity;
+       }
+
+       /*
+        * If for some reason the user modifies the CPU affinity
+        * of this thread, than stop migrating for the duration
+        * of the current test.
+        */
+       if (!cpumask_equal(current_mask, &current->cpus_allowed))
+               goto disable;
+
+       get_online_cpus();
+       cpumask_and(current_mask, cpu_online_mask, tracing_buffer_mask);
+       next_cpu = cpumask_next(smp_processor_id(), current_mask);
+       put_online_cpus();
+
+       if (next_cpu >= nr_cpu_ids)
+               next_cpu = cpumask_first(current_mask);
+
+ set_affinity:
+       if (next_cpu >= nr_cpu_ids) /* Shouldn't happen! */
+               goto disable;
+
+       cpumask_clear(current_mask);
+       cpumask_set_cpu(next_cpu, current_mask);
+
+       sched_setaffinity(0, current_mask);
+       return;
+
+ disable:
+       disable_migrate = true;
+}
+
 /*
  * kthread_fn - The CPU time sampling/hardware latency detection kernel thread
  *
@@ -230,6 +282,8 @@ static int kthread_fn(void *data)
 
        while (!kthread_should_stop()) {
 
+               move_to_next_cpu();
+
                local_irq_disable();
                get_sample();
                local_irq_enable();
@@ -473,6 +527,7 @@ static int hwlat_tracer_init(struct trace_array *tr)
 
        hwlat_trace = tr;
 
+       disable_migrate = false;
        hwlat_data.count = 0;
        tr->max_latency = 0;
        save_tracing_thresh = tracing_thresh;