sched: Make task->start_time nanoseconds based

Simplify the timespec to nsec/usec conversions.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>

include/linux/sched.h

@@ -1367,7 +1367,7 @@ struct task_struct {
 	} vtime_snap_whence;
 #endif
 	unsigned long nvcsw, nivcsw; /* context switch counts */
-	struct timespec start_time; 		/* monotonic time */
+	u64 start_time;		/* monotonic time in nsec */
 	u64 real_start_time;	/* boot based time in nsec */
 /* mm fault and swap info: this can arguably be seen as either mm-specific or thread-specific */
 	unsigned long min_flt, maj_flt;

kernel/acct.c

@@ -458,9 +458,7 @@ static void do_acct_process(struct bsd_acct_struct *acct,
 	acct_t ac;
 	mm_segment_t fs;
 	unsigned long flim;
-	u64 elapsed;
-	u64 run_time;
-	struct timespec uptime;
+	u64 elapsed, run_time;
 	struct tty_struct *tty;
 	const struct cred *orig_cred;
 
@@ -484,10 +482,8 @@ static void do_acct_process(struct bsd_acct_struct *acct,
 	strlcpy(ac.ac_comm, current->comm, sizeof(ac.ac_comm));
 
 	/* calculate run_time in nsec*/
-	ktime_get_ts(&uptime);
-	run_time = (u64)uptime.tv_sec*NSEC_PER_SEC + uptime.tv_nsec;
-	run_time -= (u64)current->group_leader->start_time.tv_sec * NSEC_PER_SEC
-		       + current->group_leader->start_time.tv_nsec;
+	run_time = ktime_get_ns();
+	run_time -= current->group_leader->start_time;
 	/* convert nsec -> AHZ */
 	elapsed = nsec_to_AHZ(run_time);
 #if ACCT_VERSION==3

kernel/fork.c

@@ -1262,7 +1262,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 
 	posix_cpu_timers_init(p);
 
-	ktime_get_ts(&p->start_time);
+	p->start_time = ktime_get_ns();
 	p->real_start_time = ktime_get_boot_ns();
 	p->io_context = NULL;
 	p->audit_context = NULL;

kernel/tsacct.c

@@ -31,20 +31,19 @@ void bacct_add_tsk(struct user_namespace *user_ns,
 		   struct taskstats *stats, struct task_struct *tsk)
 {
 	const struct cred *tcred;
-	struct timespec uptime, ts;
 	cputime_t utime, stime, utimescaled, stimescaled;
-	u64 ac_etime;
+	u64 delta;
 
 	BUILD_BUG_ON(TS_COMM_LEN < TASK_COMM_LEN);
 
-	/* calculate task elapsed time in timespec */
-	ktime_get_ts(&uptime);
-	ts = timespec_sub(uptime, tsk->start_time);
-	/* rebase elapsed time to usec (should never be negative) */
-	ac_etime = timespec_to_ns(&ts);
-	do_div(ac_etime, NSEC_PER_USEC);
-	stats->ac_etime = ac_etime;
-	stats->ac_btime = get_seconds() - ts.tv_sec;
+	/* calculate task elapsed time in nsec */
+	delta = ktime_get_ns() - tsk->start_time;
+	/* Convert to micro seconds */
+	do_div(delta, NSEC_PER_USEC);
+	stats->ac_etime = delta;
+	/* Convert to seconds for btime */
+	do_div(delta, USEC_PER_SEC);
+	stats->ac_btime = get_seconds() - delta;
 	if (thread_group_leader(tsk)) {
 		stats->ac_exitcode = tsk->exit_code;
 		if (tsk->flags & PF_FORKNOEXEC)