Add tracing support.

(KConfig still missing.)

fix sched_trace printk hooks (to be merged)

Fix feather-trace generic impl. (to be merged)

5 files changed, 821 insertions, 0 deletions

diff --git a/include/asm-x86/feather_trace.h b/include/asm-x86/feather_trace.h
new file mode 100644
index 000000000000..f60fbed07afb
--- /dev/null
+++ b/include/asm-x86/feather_trace.h
@@ -0,0 +1,11 @@
+#ifndef _ARCH_FEATHER_TRACE_H
+#define _ARCH_FEATHER_TRACE_H
+
+static inline unsigned long long ft_timestamp(void)
+{
+        unsigned long long ret;
+        __asm__ __volatile__("rdtsc" : "=A" (ret));
+        return ret;
+}
+
+#endif
diff --git a/include/litmus/feather_trace.h b/include/litmus/feather_trace.h
index 3ac1ee5e0277..eef8af7a414e 100644
--- a/include/litmus/feather_trace.h
+++ b/include/litmus/feather_trace.h
@@ -1,6 +1,7 @@
 #ifndef _FEATHER_TRACE_H_
 #define _FEATHER_TRACE_H_
 
+#include <asm/feather_trace.h>
 
 int ft_enable_event(unsigned long id);
 int ft_disable_event(unsigned long id);
@@ -30,6 +31,19 @@ extern int ft_events[MAX_EVENTS];
 
 #define ft_event3(id, callback, p, p2, p3) \
         if (ft_events[id]) callback(id, p, p2, p3);
+
+#include <asm/atomic.h>
+
+static inline int fetch_and_inc(int *val)
+{
+        return atomic_add_return(1, (atomic_t*) val) - 1;
+}
+
+static inline int fetch_and_dec(int *val)
+{
+        return atomic_sub_return(1, (atomic_t*) val) + 1;
+}
+
 #endif
 
 
diff --git a/litmus/sched_task_trace.c b/litmus/sched_task_trace.c
new file mode 100644
index 000000000000..c9dbfb22c2c6
--- /dev/null
+++ b/litmus/sched_task_trace.c
@@ -0,0 +1,200 @@
+/* sched_task_trace.c -- record scheduling events to a byte stream
+ *
+ */
+
+#define NO_TASK_TRACE_DECLS
+
+#include <linux/module.h>
+#include <linux/sched.h>
+#include <linux/percpu.h>
+
+#include <litmus/ftdev.h>
+#include <litmus/litmus.h>
+
+#include <litmus/sched_trace.h>
+#include <litmus/feather_trace.h>
+#include <litmus/ftdev.h>
+
+#define FT_TASK_TRACE_MAJOR     253
+#define NO_EVENTS               4096 /* this is a buffer of 12 4k pages per CPU */
+
+#define now() litmus_clock()
+
+struct local_buffer {
+        struct st_event_record record[NO_EVENTS];
+        char   flag[NO_EVENTS];
+        struct ft_buffer ftbuf;
+};
+
+DEFINE_PER_CPU(struct local_buffer, st_event_buffer);
+
+static struct ftdev st_dev;
+
+static int st_dev_can_open(struct ftdev *dev, unsigned int cpu)
+{
+        return cpu_online(cpu) ? 0 : -ENODEV;
+}
+
+static int __init init_sched_task_trace(void)
+{
+        struct local_buffer* buf;
+        int i, ok = 0;
+        ftdev_init(&st_dev, THIS_MODULE);
+        for (i = 0; i < NR_CPUS; i++) {
+                buf = &per_cpu(st_event_buffer, i);
+                ok += init_ft_buffer(&buf->ftbuf, NO_EVENTS,
+                                     sizeof(struct st_event_record),
+                                     buf->flag,
+                                     buf->record);
+                st_dev.minor[i].buf = &buf->ftbuf;
+        }
+        if (ok == NR_CPUS) {
+                st_dev.minor_cnt = NR_CPUS;
+                st_dev.can_open = st_dev_can_open;
+                return register_ftdev(&st_dev, "sched_trace", FT_TASK_TRACE_MAJOR);
+        } else
+                return -EINVAL;
+}
+
+module_init(init_sched_task_trace);
+
+
+static inline struct st_event_record* get_record(u8 type, struct task_struct* t)
+{
+        struct st_event_record* rec;
+        struct local_buffer* buf;
+
+        buf = &get_cpu_var(st_event_buffer);
+        if (ft_buffer_start_write(&buf->ftbuf, (void**) &rec)) {
+                rec->hdr.type = type;
+                rec->hdr.cpu  = smp_processor_id();
+                rec->hdr.pid  = t ? t->pid : 0;
+                rec->hdr.job  = t ? t->rt_param.job_params.job_no : 0;
+        } else
+                put_cpu_var(st_event_buffer);
+        /* rec will be NULL if it failed */
+        return rec;
+}
+
+static inline void put_record(struct st_event_record* rec)
+{
+        struct local_buffer* buf;
+        buf = &__get_cpu_var(st_event_buffer);
+        ft_buffer_finish_write(&buf->ftbuf, rec);
+        put_cpu_var(st_event_buffer);
+}
+
+feather_callback void do_sched_trace_task_name(unsigned long id, unsigned long _task)
+{
+        struct task_struct *t = (struct task_struct*) _task;
+        struct st_event_record* rec = get_record(ST_NAME, t);
+        int i;
+        if (rec) {
+                for (i = 0; i < min(TASK_COMM_LEN, ST_NAME_LEN); i++)
+                        rec->data.name.cmd[i] = t->comm[i];
+                put_record(rec);
+        }
+}
+
+feather_callback void do_sched_trace_task_param(unsigned long id, unsigned long _task)
+{
+        struct task_struct *t = (struct task_struct*) _task;
+        struct st_event_record* rec = get_record(ST_PARAM, t);
+        if (rec) {
+                rec->data.param.wcet      = get_exec_cost(t);
+                rec->data.param.period    = get_rt_period(t);
+                rec->data.param.phase     = get_rt_phase(t);
+                rec->data.param.partition = get_partition(t);
+                put_record(rec);
+        }
+}
+
+feather_callback void do_sched_trace_task_release(unsigned long id, unsigned long _task)
+{
+        struct task_struct *t = (struct task_struct*) _task;
+        struct st_event_record* rec = get_record(ST_RELEASE, t);
+        if (rec) {
+                rec->data.release.release  = get_release(t);
+                rec->data.release.deadline = get_deadline(t);
+                put_record(rec);
+        }
+}
+
+/* skipped: st_assigned_data, we don't use it atm */
+
+feather_callback void do_sched_trace_task_switch_to(unsigned long id,
+                                                    unsigned long _task)
+{
+        struct task_struct *t = (struct task_struct*) _task;
+        struct st_event_record* rec;
+        if (is_realtime(t)) {
+                rec = get_record(ST_SWITCH_TO, t);
+                if (rec) {
+                        rec->data.switch_to.when      = now();
+                        rec->data.switch_to.exec_time = get_exec_time(t);
+                        put_record(rec);
+                }
+        }
+}
+
+feather_callback void do_sched_trace_task_switch_away(unsigned long id,
+                                                      unsigned long _task)
+{
+        struct task_struct *t = (struct task_struct*) _task;
+        struct st_event_record* rec;
+        if (is_realtime(t)) {
+                rec = get_record(ST_SWITCH_AWAY, t);
+                if (rec) {
+                        rec->data.switch_away.when      = now();
+                        rec->data.switch_away.exec_time = get_exec_time(t);
+                        put_record(rec);
+                }
+        }
+}
+
+feather_callback void do_sched_trace_task_completion(unsigned long id,
+                                                     unsigned long _task,
+                                                     unsigned long forced)
+{
+        struct task_struct *t = (struct task_struct*) _task;
+        struct st_event_record* rec = get_record(ST_COMPLETION, t);
+        if (rec) {
+                rec->data.completion.when   = now();
+                rec->data.completion.forced = forced;
+                put_record(rec);
+        }
+}
+
+feather_callback void do_sched_trace_task_block(unsigned long id,
+                                                unsigned long _task)
+{
+        struct task_struct *t = (struct task_struct*) _task;
+        struct st_event_record* rec = get_record(ST_BLOCK, t);
+        if (rec) {
+                rec->data.block.when      = now();
+                put_record(rec);
+        }
+}
+
+feather_callback void do_sched_trace_task_resume(unsigned long id,
+                                                 unsigned long _task)
+{
+        struct task_struct *t = (struct task_struct*) _task;
+        struct st_event_record* rec = get_record(ST_RESUME, t);
+        if (rec) {
+                rec->data.resume.when      = now();
+                put_record(rec);
+        }
+}
+
+feather_callback void do_sched_trace_sys_release(unsigned long id,
+                                                 unsigned long _start)
+{
+        lt_t *start = (lt_t*) _start;
+        struct st_event_record* rec = get_record(ST_SYS_RELEASE, NULL);
+        if (rec) {
+                rec->data.sys_release.when    = now();
+                rec->data.sys_release.release = *start;
+                put_record(rec);
+        }
+}
diff --git a/litmus/sched_trace.c b/litmus/sched_trace.c
new file mode 100644
index 000000000000..8581c03bb137
--- /dev/null
+++ b/litmus/sched_trace.c
@@ -0,0 +1,498 @@
+/* sched_trace.c -- record scheduling events to a byte stream.
+ *
+ * TODO: Move ring buffer to a lockfree implementation.
+ */
+
+#include <linux/spinlock.h>
+#include <linux/fs.h>
+#include <linux/cdev.h>
+#include <asm/semaphore.h>
+#include <asm/uaccess.h>
+#include <linux/module.h>
+#include <linux/sysrq.h>
+
+#include <litmus/sched_trace.h>
+#include <litmus/litmus.h>
+
+typedef struct {
+        /*      guard read and write pointers                   */
+        spinlock_t      lock;
+        /*      guard against concurrent freeing of buffer      */
+        rwlock_t        del_lock;
+
+        /*      memory allocated for ring buffer                */
+        unsigned long   order;
+        char*           buf;
+        char*           end;
+
+        /*      Read/write pointer. May not cross.
+         *      They point to the position of next write and
+         *      last read.
+         */
+        char*           writep;
+        char*           readp;
+
+} ring_buffer_t;
+
+#define EMPTY_RING_BUFFER {     \
+        .lock     = SPIN_LOCK_UNLOCKED,         \
+        .del_lock = RW_LOCK_UNLOCKED,           \
+        .buf      = NULL,                       \
+        .end      = NULL,                       \
+        .writep   = NULL,                       \
+        .readp    = NULL                        \
+}
+
+void rb_init(ring_buffer_t* buf)
+{
+        *buf = (ring_buffer_t) EMPTY_RING_BUFFER;
+}
+
+int rb_alloc_buf(ring_buffer_t* buf, unsigned long order)
+{
+        unsigned long flags;
+        int error = 0;
+        char *mem;
+
+        /* do memory allocation while not atomic */
+        mem = (char *) __get_free_pages(GFP_KERNEL, order);
+        if (!mem)
+                return -ENOMEM;
+        write_lock_irqsave(&buf->del_lock, flags);
+        BUG_ON(buf->buf);
+        buf->buf = mem;
+        buf->end = buf->buf + PAGE_SIZE * (1 << order) - 1;
+        memset(buf->buf, 0xff, buf->end - buf->buf);
+        buf->order = order;
+        buf->writep = buf->buf + 1;
+        buf->readp  = buf->buf;
+        write_unlock_irqrestore(&buf->del_lock, flags);
+        return error;
+}
+
+int rb_free_buf(ring_buffer_t* buf)
+{
+        unsigned long flags;
+        int error = 0;
+        write_lock_irqsave(&buf->del_lock, flags);
+        BUG_ON(!buf->buf);
+        free_pages((unsigned long) buf->buf, buf->order);
+        buf->buf    = NULL;
+        buf->end    = NULL;
+        buf->writep = NULL;
+        buf->readp  = NULL;
+        write_unlock_irqrestore(&buf->del_lock, flags);
+        return error;
+}
+
+/* Assumption: concurrent writes are serialized externally
+ *
+ * Will only succeed if there is enough space for all len bytes.
+ */
+int rb_put(ring_buffer_t* buf, char* mem, size_t len)
+{
+        unsigned long flags;
+        char* r , *w;
+        int error = 0;
+        read_lock_irqsave(&buf->del_lock, flags);
+        if (!buf->buf) {
+                error = -ENODEV;
+                goto out;
+        }
+        spin_lock(&buf->lock);
+        r = buf->readp;
+        w = buf->writep;
+        spin_unlock(&buf->lock);
+        if (r < w && buf->end - w >= len - 1) {
+                /* easy case: there is enough space in the buffer
+                 * to write it in one continous chunk*/
+                memcpy(w, mem, len);
+                w += len;
+                if (w > buf->end)
+                        /* special case: fit exactly into buffer
+                         * w is now buf->end + 1
+                         */
+                        w = buf->buf;
+        } else if (w < r && r - w >= len) { /* >= len because  may not cross */
+                /* we are constrained by the read pointer but we there
+                 * is enough space
+                 */
+                memcpy(w, mem, len);
+                w += len;
+        } else if (r <= w && buf->end - w < len - 1) {
+                /* the wrap around case: there may or may not be space */
+                if ((buf->end - w) + (r - buf->buf) >= len - 1) {
+                        /* copy chunk that fits at the end */
+                        memcpy(w, mem, buf->end - w + 1);
+                        mem += buf->end - w + 1;
+                        len -= (buf->end - w + 1);
+                        w = buf->buf;
+                        /* copy the rest */
+                        memcpy(w, mem, len);
+                        w += len;
+                }
+                else
+                        error = -ENOMEM;
+        } else {
+                error = -ENOMEM;
+        }
+        if (!error) {
+                spin_lock(&buf->lock);
+                buf->writep = w;
+                spin_unlock(&buf->lock);
+        }
+ out:
+        read_unlock_irqrestore(&buf->del_lock, flags);
+        return error;
+}
+
+/* Assumption: concurrent reads are serialized externally */
+int rb_get(ring_buffer_t* buf, char* mem, size_t len)
+{
+        unsigned long flags;
+        char* r , *w;
+        int error = 0;
+        read_lock_irqsave(&buf->del_lock, flags);
+        if (!buf->buf) {
+                error = -ENODEV;
+                goto out;
+        }
+        spin_lock(&buf->lock);
+        r = buf->readp;
+        w = buf->writep;
+        spin_unlock(&buf->lock);
+
+        if (w <= r && buf->end - r >= len) {
+                /* easy case: there is enough data in the buffer
+                 * to get it in one  chunk*/
+                memcpy(mem, r + 1, len);
+                r += len;
+                error = len;
+
+        } else if (r + 1 < w && w - r - 1 >= len) {
+                /* we are constrained by the write pointer but
+                 * there is enough data
+                 */
+                memcpy(mem, r + 1, len);
+                r += len;
+                error = len;
+
+        } else if (r + 1 < w && w - r - 1 < len) {
+                /* we are constrained by the write pointer and there
+                 * there is not enough data
+                 */
+                memcpy(mem, r + 1, w - r - 1);
+                error = w - r - 1;
+                r    += w - r - 1;
+
+        } else if (w <= r && buf->end - r < len) {
+                /* the wrap around case: there may or may not be enough data
+                 * first let's get what is available
+                 */
+                memcpy(mem, r + 1, buf->end - r);
+                error += (buf->end - r);
+                mem   += (buf->end - r);
+                len   -= (buf->end - r);
+                r     += (buf->end - r);
+
+                if (w > buf->buf) {
+                        /* there is more to get */
+                        r = buf->buf - 1;
+                        if (w - r >= len) {
+                                /* plenty */
+                                memcpy(mem, r + 1, len);
+                                error += len;
+                                r     += len;
+                        } else {
+                                memcpy(mem, r + 1, w - r - 1);
+                                error += w - r - 1;
+                                r     += w - r - 1;
+                        }
+                }
+        } /* nothing available */
+
+        if (error > 0) {
+                spin_lock(&buf->lock);
+                buf->readp = r;
+                spin_unlock(&buf->lock);
+        }
+ out:
+        read_unlock_irqrestore(&buf->del_lock, flags);
+        return error;
+}
+
+
+
+/******************************************************************************/
+/*                        DEVICE FILE DRIVER                                  */
+/******************************************************************************/
+
+
+
+/* Allocate a buffer of about 1 MB per CPU.
+ *
+ */
+#define BUFFER_ORDER 8
+
+typedef struct {
+        ring_buffer_t           buf;
+        atomic_t                reader_cnt;
+        struct semaphore        reader_mutex;
+} trace_buffer_t;
+
+
+/* This does not initialize the semaphore!! */
+
+#define EMPTY_TRACE_BUFFER \
+        { .buf = EMPTY_RING_BUFFER, .reader_cnt = ATOMIC_INIT(0)}
+
+static spinlock_t               log_buffer_lock = SPIN_LOCK_UNLOCKED;
+static trace_buffer_t           log_buffer = EMPTY_TRACE_BUFFER;
+
+static void init_log_buffer(void)
+{
+        /* only initialize the mutex, the rest was initialized as part
+         * of the static initialization macro
+         */
+        init_MUTEX(&log_buffer.reader_mutex);
+}
+
+static ssize_t log_read(struct file *filp, char __user *to, size_t len,
+                      loff_t *f_pos)
+{
+        /*      we ignore f_pos, this is strictly sequential */
+
+        ssize_t error = -EINVAL;
+        char*   mem;
+        trace_buffer_t *buf = filp->private_data;
+
+        if (down_interruptible(&buf->reader_mutex)) {
+                error = -ERESTARTSYS;
+                goto out;
+        }
+
+        if (len > 64 * 1024)
+                len = 64 * 1024;
+        mem = kmalloc(len, GFP_KERNEL);
+        if (!mem) {
+                error = -ENOMEM;
+                goto out_unlock;
+        }
+
+        error = rb_get(&buf->buf, mem, len);
+        while (!error) {
+                set_current_state(TASK_INTERRUPTIBLE);
+                schedule_timeout(110);
+                if (signal_pending(current))
+                        error = -ERESTARTSYS;
+                else
+                        error = rb_get(&buf->buf, mem, len);
+        }
+
+        if (error > 0 && copy_to_user(to, mem, error))
+                error = -EFAULT;
+
+        kfree(mem);
+ out_unlock:
+        up(&buf->reader_mutex);
+ out:
+        return error;
+}
+
+
+
+
+/* log_open - open the global log message ring buffer.
+ */
+static int log_open(struct inode *in, struct file *filp)
+{
+        int error               = -EINVAL;
+        trace_buffer_t* buf;
+
+        buf = &log_buffer;
+
+        if (down_interruptible(&buf->reader_mutex)) {
+                error = -ERESTARTSYS;
+                goto out;
+        }
+
+        /*      first open must allocate buffers        */
+        if (atomic_inc_return(&buf->reader_cnt) == 1) {
+                if ((error = rb_alloc_buf(&buf->buf, BUFFER_ORDER)))
+                {
+                        atomic_dec(&buf->reader_cnt);
+                        goto out_unlock;
+                }
+        }
+
+        error = 0;
+        filp->private_data = buf;
+        printk(KERN_DEBUG "sched_trace buf: from 0x%p to 0x%p  length: %x\n",
+               buf->buf.buf, buf->buf.end,
+               (unsigned int) (buf->buf.end - buf->buf.buf));
+ out_unlock:
+        up(&buf->reader_mutex);
+ out:
+        return error;
+}
+
+static int log_release(struct inode *in, struct file *filp)
+{
+        int error               = -EINVAL;
+        trace_buffer_t* buf     = filp->private_data;
+
+        BUG_ON(!filp->private_data);
+
+        if (down_interruptible(&buf->reader_mutex)) {
+                error = -ERESTARTSYS;
+                goto out;
+        }
+
+        /*      last release must deallocate buffers    */
+        if (atomic_dec_return(&buf->reader_cnt) == 0) {
+                error = rb_free_buf(&buf->buf);
+        }
+
+        up(&buf->reader_mutex);
+ out:
+        return error;
+}
+
+/******************************************************************************/
+/*                          Device Registration                               */
+/******************************************************************************/
+
+/* the major numbes are from the unassigned/local use block
+ *
+ * This should be converted to dynamic allocation at some point...
+ */
+#define LOG_MAJOR       251
+
+/* log_fops  - The file operations for accessing the global LITMUS log message
+ *             buffer.
+ *
+ * Except for opening the device file it uses the same operations as trace_fops.
+ */
+struct file_operations log_fops = {
+        .owner   = THIS_MODULE,
+        .open    = log_open,
+        .release = log_release,
+        .read    = log_read,
+};
+
+static int __init register_buffer_dev(const char* name,
+                                      struct file_operations* fops,
+                                      int major, int count)
+{
+        dev_t  trace_dev;
+        struct cdev *cdev;
+        int error = 0;
+
+        trace_dev = MKDEV(major, 0);
+        error     = register_chrdev_region(trace_dev, count, name);
+        if (error)
+        {
+                printk(KERN_WARNING "sched trace: "
+                       "Could not register major/minor number %d\n", major);
+                return error;
+        }
+        cdev = cdev_alloc();
+        if (!cdev) {
+                printk(KERN_WARNING "sched trace: "
+                        "Could not get a cdev for %s.\n", name);
+                return -ENOMEM;
+        }
+        cdev->owner = THIS_MODULE;
+        cdev->ops   = fops;
+        error = cdev_add(cdev, trace_dev, count);
+        if (error) {
+                printk(KERN_WARNING "sched trace: "
+                        "add_cdev failed for %s.\n", name);
+                return -ENOMEM;
+        }
+        return error;
+
+}
+
+#ifdef CONFIG_MAGIC_SYSRQ
+
+static void sysrq_dump_trace_buffer(int key, struct tty_struct *tty)
+{
+        dump_trace_buffer(100);
+}
+
+static struct sysrq_key_op sysrq_dump_trace_buffer_op = {
+        .handler        = sysrq_dump_trace_buffer,
+        .help_msg       = "dump-trace-buffer(Y)",
+        .action_msg     = "writing content of TRACE() buffer",
+};
+
+#endif
+
+static int __init init_sched_trace(void)
+{
+        printk("Initializing TRACE() device\n");
+        init_log_buffer();
+
+#ifdef CONFIG_MAGIC_SYSRQ
+        /* offer some debugging help */
+        if (!register_sysrq_key('y', &sysrq_dump_trace_buffer_op))
+                printk("Registered dump-trace-buffer(Y) magic sysrq.\n");
+        else
+                printk("Could not register dump-trace-buffer(Y) magic sysrq.\n");
+#endif
+
+
+        return register_buffer_dev("litmus_log", &log_fops,
+                                   LOG_MAJOR, 1);
+}
+
+module_init(init_sched_trace);
+
+#define MSG_SIZE 255
+static DEFINE_PER_CPU(char[MSG_SIZE], fmt_buffer);
+
+/* sched_trace_log_message - This is the only function that accesses the the
+ *                           log buffer inside the kernel for writing.
+ *                           Concurrent access to it is serialized via the
+ *                           log_buffer_lock.
+ *
+ *                           The maximum length of a formatted message is 255.
+ */
+void sched_trace_log_message(const char* fmt, ...)
+{
+        unsigned long   flags;
+        va_list         args;
+        size_t          len;
+        char*           buf;
+
+        va_start(args, fmt);
+        local_irq_save(flags);
+
+        /* format message */
+        buf = __get_cpu_var(fmt_buffer);
+        len = vscnprintf(buf, MSG_SIZE, fmt, args);
+
+        spin_lock(&log_buffer_lock);
+        /* Don't copy the trailing null byte, we don't want null bytes
+         * in a text file.
+         */
+        rb_put(&log_buffer.buf, buf, len);
+        spin_unlock(&log_buffer_lock);
+
+        local_irq_restore(flags);
+        va_end(args);
+}
+
+void dump_trace_buffer(int max)
+{
+        char line[80];
+        int len;
+        int count = 0;
+
+        while ((max == 0 || count++ < max) &&
+               (len = rb_get(&log_buffer.buf, line, sizeof(line) - 1)) > 0) {
+                line[len] = '\0';
+                printk("%s", line);
+        }
+}
diff --git a/litmus/trace.c b/litmus/trace.c
new file mode 100644
index 000000000000..6a134121e18c
--- /dev/null
+++ b/litmus/trace.c
@@ -0,0 +1,98 @@
+#include <linux/module.h>
+
+#include <litmus/ftdev.h>
+#include <litmus/litmus.h>
+#include <litmus/trace.h>
+
+/******************************************************************************/
+/*                          Allocation                                        */
+/******************************************************************************/
+
+static struct ftdev overhead_dev;
+
+#define trace_ts_buf overhead_dev.minor[0].buf
+
+static unsigned int ts_seq_no = 0;
+
+static inline void __save_timestamp_cpu(unsigned long event,
+                                        uint8_t type, uint8_t cpu)
+{
+        unsigned int seq_no;
+        struct timestamp *ts;
+        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;
+                ft_buffer_finish_write(trace_ts_buf, ts);
+        }
+}
+
+static inline void __save_timestamp(unsigned long event,
+                                   uint8_t type)
+{
+        __save_timestamp_cpu(event, type, raw_smp_processor_id());
+}
+
+feather_callback void save_timestamp(unsigned long event)
+{
+        __save_timestamp(event, TSK_UNKNOWN);
+}
+
+feather_callback void save_timestamp_def(unsigned long event,
+                                         unsigned long type)
+{
+        __save_timestamp(event, (uint8_t) type);
+}
+
+feather_callback void save_timestamp_task(unsigned long event,
+                                          unsigned long t_ptr)
+{
+        int rt = is_realtime((struct task_struct *) t_ptr);
+        __save_timestamp(event, rt ? TSK_RT : TSK_BE);
+}
+
+feather_callback void save_timestamp_cpu(unsigned long event,
+                                         unsigned long cpu)
+{
+        __save_timestamp_cpu(event, TSK_UNKNOWN, cpu);
+}
+
+/******************************************************************************/
+/*                        DEVICE FILE DRIVER                                  */
+/******************************************************************************/
+
+#define NO_TIMESTAMPS (2 << 19) /* that should be 8 megs of ram, we may not get
+                                 * as much */
+#define FT_TRACE_MAJOR  252
+
+static int alloc_timestamp_buffer(struct ftdev* ftdev, unsigned int idx)
+{
+        unsigned int count = NO_TIMESTAMPS;
+        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));
+                count /= 2;
+        }
+        return ftdev->minor[idx].buf ? 0 : -ENOMEM;
+}
+
+static void free_timestamp_buffer(struct ftdev* ftdev, unsigned int idx)
+{
+        free_ft_buffer(ftdev->minor[idx].buf);
+        ftdev->minor[idx].buf = NULL;
+}
+
+static int __init init_ft_overhead_trace(void)
+{
+        printk("Initializing Feather-Trace overhead tracing device.\n");
+        ftdev_init(&overhead_dev, THIS_MODULE);
+        overhead_dev.minor_cnt = 1; /* only one buffer */
+        overhead_dev.alloc = alloc_timestamp_buffer;
+        overhead_dev.free  = free_timestamp_buffer;
+        return register_ftdev(&overhead_dev, "ft_trace", FT_TRACE_MAJOR);
+}
+
+module_init(init_ft_overhead_trace);