On Fri, Jan 29, 2016 at 10:36:02PM -0500, [email protected] wrote: > From: Rik van Riel <[email protected]> > > 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 <[email protected]> > --- > 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);
You might want to add a comment specifying why we don't call cputime_to_usecs() directly (because we optimize if delta < TICK_NSEC). Although this has a good impact on nohz_full, it might have a tiny bad one on !nohz_full because now we first convert jiffies to nsecs (which implies a multiplication by 1000) that we later divide again by 1000. Now this is ok because I plan to convert tsk->utime/stime to nsecs and thus remove most of the cputime_t use and conversions everywhere. > > - 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; The use of 1024 and the change on MB above are confusing me. Why are we doing that? Thanks. > out: > local_irq_restore(flags); > } > -- > 2.5.0 >
