sched_trace: make buffer size configurable

Large sched_trace buffers cause boot problems on the ARM box. Allow
the user to specify smaller buffers.

2 files changed, 24 insertions, 1 deletions

diff --git a/litmus/Kconfig b/litmus/Kconfig
index 9888589ef126..f1de6fabfc17 100644
--- a/litmus/Kconfig
+++ b/litmus/Kconfig
@@ -108,6 +108,26 @@ config SCHED_TASK_TRACE
           Say Yes for debugging.
           Say No for overhead tracing.
 
+config SCHED_TASK_TRACE_SHIFT
+       int "Buffer size for sched_trace_xxx() events"
+       depends on SCHED_TASK_TRACE
+       range 8 13
+       default 9
+       help
+
+         Select the buffer size of sched_trace_xxx() events as a power of two.
+         These buffers are statically allocated as per-CPU data. Each event
+         requires 24 bytes storage plus one additional flag byte. Too large
+         buffers can cause issues with the per-cpu allocator (and waste
+         memory). Too small buffers can cause scheduling events to be lost. The
+         "right" size is workload dependent and depends on the number of tasks,
+         each task's period, each task's number of suspensions, and how often
+         the buffer is flushed.
+
+         Examples: 12 =>   4k events
+                   10 =>   1k events
+                    8 =>  512 events
+
 config SCHED_OVERHEAD_TRACE
         bool "Record timestamps for overhead measurements"
         depends on FEATHER_TRACE
diff --git a/litmus/sched_task_trace.c b/litmus/sched_task_trace.c
index 39a543e22d41..5fcabb55eb8f 100644
--- a/litmus/sched_task_trace.c
+++ b/litmus/sched_task_trace.c
@@ -18,7 +18,7 @@
 
 /* set MAJOR to 0 to have it dynamically assigned */
 #define FT_TASK_TRACE_MAJOR     253
-#define NO_EVENTS               4096 /* this is a buffer of 12 4k pages per CPU */
+#define NO_EVENTS               (1 << CONFIG_SCHED_TASK_TRACE_SHIFT)
 
 #define now() litmus_clock()
 
@@ -41,6 +41,9 @@ static int __init init_sched_task_trace(void)
 {
         struct local_buffer* buf;
         int i, ok = 0;
+        printk("Allocated %u sched_trace_xxx() events per CPU "
+               "(buffer size: %u bytes)\n",
+               NO_EVENTS, sizeof(struct local_buffer));
         ftdev_init(&st_dev, THIS_MODULE);
         for (i = 0; i < NR_CPUS; i++) {
                 buf = &per_cpu(st_event_buffer, i);