sched, time: Remove non-power-of-two divides from __acct_update_integrals()

When running a microbenchmark calling an invalid syscall number
in a loop, on a nohz_full CPU, we spend a full 9% of our CPU
time in __acct_update_integrals().

This function converts cputime_t to jiffies, to a timeval, only to
convert the timeval back to microseconds before discarding it.

This patch leaves __acct_update_integrals() functionally equivalent,
but speeds things up by about 12%, with 10 million calls to an
invalid syscall number dropping from 3.7 to 3.25 seconds.

Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: clark@redhat.com
Cc: eric.dumazet@gmail.com
Cc: fweisbec@gmail.com
Cc: luto@amacapital.net
Link: http://lkml.kernel.org/r/1455152907-18495-2-git-send-email-riel@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/tsacct.c

@@ -93,9 +93,11 @@ void xacct_add_tsk(struct taskstats *stats, struct task_struct *p)
 {
 	struct mm_struct *mm;
 
-	/* convert pages-usec to Mbyte-usec */
-	stats->coremem = p->acct_rss_mem1 * PAGE_SIZE / MB;
-	stats->virtmem = p->acct_vm_mem1 * PAGE_SIZE / MB;
+	/* convert pages-nsec/1024 to Mbyte-usec, see __acct_update_integrals */
+	stats->coremem = p->acct_rss_mem1 * PAGE_SIZE;
+	do_div(stats->coremem, 1000 * KB);
+	stats->virtmem = p->acct_vm_mem1 * PAGE_SIZE;
+	do_div(stats->virtmem, 1000 * KB);
 	mm = get_task_mm(p);
 	if (mm) {
 		/* adjust to KB unit */
@@ -125,22 +127,26 @@ static void __acct_update_integrals(struct task_struct *tsk,
 {
 	if (likely(tsk->mm)) {
 		cputime_t time, dtime;
-		struct timeval value;
 		unsigned long flags;
 		u64 delta;
 
 		local_irq_save(flags);
 		time = stime + utime;
 		dtime = time - tsk->acct_timexpd;
-		jiffies_to_timeval(cputime_to_jiffies(dtime), &value);
-		delta = value.tv_sec;
-		delta = delta * USEC_PER_SEC + value.tv_usec;
+		/* Avoid division: cputime_t is often in nanoseconds already. */
+		delta = cputime_to_nsecs(dtime);
 
-		if (delta == 0)
+		if (delta < TICK_NSEC)
 			goto out;
+
 		tsk->acct_timexpd = time;
-		tsk->acct_rss_mem1 += delta * get_mm_rss(tsk->mm);
-		tsk->acct_vm_mem1 += delta * tsk->mm->total_vm;
+		/*
+		 * Divide by 1024 to avoid overflow, and to avoid division.
+		 * The final unit reported to userspace is Mbyte-usecs,
+		 * the rest of the math is done in xacct_add_tsk.
+		 */
+		tsk->acct_rss_mem1 += delta * get_mm_rss(tsk->mm) >> 10;
+		tsk->acct_vm_mem1 += delta * tsk->mm->total_vm >> 10;
 	out:
 		local_irq_restore(flags);
 	}