[PATCH 1/4] sched,time: remove non-power-of-two divides from __acct_update_integrals

[riel]

From: Rik van Riel <r...@redhat.com>

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 <r...@redhat.com>
---
 kernel/tsacct.c | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/kernel/tsacct.c b/kernel/tsacct.c
index 975cb49e32bf..41667b23dbd0 100644
--- a/kernel/tsacct.c
+++ b/kernel/tsacct.c
@@ -93,9 +93,9 @@ 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/KB to Mbyte-usec, see __acct_update_integrals */
+       stats->coremem = p->acct_rss_mem1 * PAGE_SIZE / (1000 * KB);
+       stats->virtmem = p->acct_vm_mem1 * PAGE_SIZE / (1000 * KB);
        mm = get_task_mm(p);
        if (mm) {
                /* adjust to KB unit */
@@ -125,22 +125,21 @@ 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;
+               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;
+               /* The final unit will be Mbyte-usecs, see xacct_add_tsk */
+               tsk->acct_rss_mem1 += delta * get_mm_rss(tsk->mm) / 1024;
+               tsk->acct_vm_mem1 += delta * tsk->mm->total_vm / 1024;
        out:
                local_irq_restore(flags);
        }
-- 
2.5.0