Messages in this thread | | | Subject | Re: [PATCH 1/4] sched,time: remove non-power-of-two divides from __acct_update_integrals | From | Rik van Riel <> | Date | Sat, 30 Jan 2016 12:53:09 -0500 |
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On 01/30/2016 09:44 AM, Frederic Weisbecker wrote: > On Fri, Jan 29, 2016 at 10:36:02PM -0500, riel@redhat.com wrote: >> From: Rik van Riel <riel@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 <riel@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); > > 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.
Isn't cputime_t in nanoseconds even on !nohz_full systems nowadays, due to sched_clock?
Also, a multiplication is essentially instantaneous compared to a division, which is why Peter suggested going this way around.
>> >> - 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.
So the compiler can just do a right shift in the frequently called code, and have no divide at all left in __acct_update_integrals. However, reducing the value here does seem useful for the prevention of overflows.
The divide is saved for when the statistics are read out to userspace.
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