sched, time: Remove non-power-of-two divides from __acct_update_integrals() (382c2fe9) · Commits · 戴 / test

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

+16 −10

Original line number	Diff line number	Diff line
		@@ -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);
		}

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