Feather-Trace: keep track of interrupt-related interference.wip-ft-irq-flag

Increment a processor-local counter whenever an interrupt is handled.
This allows Feather-Trace to include a (truncated) counter and a flag
to report interference from interrupts. This could be used to filter
samples that were disturbed by interrupts.

1 files changed, 45 insertions, 2 deletions

diff --git a/litmus/trace.c b/litmus/trace.c
index 5d77806da647..3c35c527e805 100644
--- a/litmus/trace.c
+++ b/litmus/trace.c
@@ -16,6 +16,35 @@ static struct ftdev overhead_dev;
 
 static unsigned int ts_seq_no = 0;
 
+DEFINE_PER_CPU(atomic_t, irq_fired_count);
+
+static inline void clear_irq_fired(void)
+{
+        atomic_set(&__raw_get_cpu_var(irq_fired_count), 0);
+}
+
+static inline unsigned int get_and_clear_irq_fired(void)
+{
+        /* This is potentially not atomic  since we might migrate if
+         * preemptions are not disabled. As a tradeoff between
+         * accuracy and tracing overheads, this seems acceptable.
+         * If it proves to be a problem, then one could add a callback
+         * from the migration code to invalidate irq_fired_count.
+         */
+        return atomic_xchg(&__raw_get_cpu_var(irq_fired_count), 0);
+}
+
+static inline void __save_irq_flags(struct timestamp *ts)
+{
+        unsigned int irq_count;
+
+        irq_count     = get_and_clear_irq_fired();
+        /* Store how many interrupts occurred. */
+        ts->irq_count = irq_count;
+        /* Extra flag because ts->irq_count overflows quickly. */
+        ts->irq_flag  = irq_count > 0;
+}
+
 static inline void __save_timestamp_cpu(unsigned long event,
                                         uint8_t type, uint8_t cpu)
 {
@@ -24,10 +53,13 @@ static inline void __save_timestamp_cpu(unsigned long event,
         seq_no = fetch_and_inc((int *) &ts_seq_no);
         if (ft_buffer_start_write(trace_ts_buf, (void**)  &ts)) {
                 ts->event     = event;
-                ts->timestamp = ft_timestamp();
                 ts->seq_no    = seq_no;
                 ts->cpu       = cpu;
                 ts->task_type = type;
+                __save_irq_flags(ts);
+                barrier();
+                /* prevent re-ordering of ft_timestamp() */
+                ts->timestamp = ft_timestamp();
                 ft_buffer_finish_write(trace_ts_buf, ts);
         }
 }
@@ -40,6 +72,7 @@ static void __add_timestamp_user(struct timestamp *pre_recorded)
         if (ft_buffer_start_write(trace_ts_buf, (void**)  &ts)) {
                 *ts = *pre_recorded;
                 ts->seq_no = seq_no;
+                __save_irq_flags(ts);
                 ft_buffer_finish_write(trace_ts_buf, ts);
         }
 }
@@ -90,6 +123,7 @@ feather_callback void save_task_latency(unsigned long event,
                 ts->seq_no    = seq_no;
                 ts->cpu       = cpu;
                 ts->task_type = TSK_RT;
+                __save_irq_flags(ts);
                 ft_buffer_finish_write(trace_ts_buf, ts);
         }
 }
@@ -107,6 +141,10 @@ feather_callback void save_task_latency(unsigned long event,
 static int alloc_timestamp_buffer(struct ftdev* ftdev, unsigned int idx)
 {
         unsigned int count = NO_TIMESTAMPS;
+
+        /* An overhead-tracing timestamp should be exactly 16 bytes long. */
+        BUILD_BUG_ON(sizeof(struct timestamp) != 16);
+
         while (count && !trace_ts_buf) {
                 printk("time stamp buffer: trying to allocate %u time stamps.\n", count);
                 ftdev->minor[idx].buf = alloc_ft_buffer(count, sizeof(struct timestamp));
@@ -149,7 +187,7 @@ out:
 
 static int __init init_ft_overhead_trace(void)
 {
-        int err;
+        int err, cpu;
 
         printk("Initializing Feather-Trace overhead tracing device.\n");
         err = ftdev_init(&overhead_dev, THIS_MODULE, 1, "ft_trace");
@@ -164,6 +202,11 @@ static int __init init_ft_overhead_trace(void)
         if (err)
                 goto err_dealloc;
 
+        /* initialize IRQ flags */
+        for (cpu = 0; cpu < NR_CPUS; cpu++)  {
+                clear_irq_fired();
+        }
+
         return 0;
 
 err_dealloc: