Make TRACE() buffer size configurable

Let the user choose an appropriate buffer size (instead of scaling
with NR_CPUS).  The kfifo api requires the buffer to be a power of
two, so enforce this constraint in the configuration.

This fixes a previously-existing compile-time error for values of
NR_CPU that are not a power of two.

Based on a patch by Mac Mollison <mollison@cs.unc.edu>.

2 files changed, 24 insertions, 4 deletions

diff --git a/litmus/Kconfig b/litmus/Kconfig
index f1de6fabfc17..a3341021f05c 100644
--- a/litmus/Kconfig
+++ b/litmus/Kconfig
@@ -149,6 +149,26 @@ config SCHED_DEBUG_TRACE
           Say Yes for debugging.
           Say No for overhead tracing.
 
+config SCHED_DEBUG_TRACE_SHIFT
+       int "Buffer size for TRACE() buffer"
+       depends on SCHED_DEBUG_TRACE
+       range 14 22
+       default 18
+       help
+
+        Select the amount of memory needed per CPU for the TRACE() buffer, as a
+        power of two. The TRACE() buffer is global and statically allocated. If
+        the buffer is too small, there will be holes in the TRACE() log if the
+        buffer-flushing task is starved.
+
+        The default should be sufficient for most systems. Increase the buffer
+        size if the log contains holes. Reduce the buffer size when running on
+        a memory-constrained system.
+
+        Examples: 14 =>  16KB
+                  18 => 256KB
+                  20 =>   1MB
+
 endmenu
 
 endmenu
diff --git a/litmus/sched_trace.c b/litmus/sched_trace.c
index 9b03e0cfe82b..f4171fddbbb1 100644
--- a/litmus/sched_trace.c
+++ b/litmus/sched_trace.c
@@ -18,14 +18,14 @@
 
 #define SCHED_TRACE_NAME "litmus/log"
 
-/* Allocate a buffer of about 32k per CPU */
-#define LITMUS_TRACE_BUF_PAGES 64
-#define LITMUS_TRACE_BUF_SIZE (PAGE_SIZE * LITMUS_TRACE_BUF_PAGES * NR_CPUS)
+/* Compute size of TRACE() buffer */
+#define LITMUS_TRACE_BUF_SIZE (1 << CONFIG_SCHED_DEBUG_TRACE_SHIFT)
 
 /* Max length of one read from the buffer */
 #define MAX_READ_LEN (64 * 1024)
 
-/* Max length for one write --- from kernel --- to the buffer */
+/* Max length for one write --- by TRACE() --- to the buffer. This is used to
+ * allocate a per-cpu buffer for printf() formatting. */
 #define MSG_SIZE 255