From: Rik van Riel <riel@redhat.com>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: linux-kernel@vger.kernel.org, tglx@linutronix.de,
mingo@kernel.org, luto@amacapital.net, peterz@infradead.org,
clark@redhat.com
Subject: Re: [PATCH 1/4] sched,time: remove non-power-of-two divides from __acct_update_integrals
Date: Sat, 30 Jan 2016 12:53:09 -0500 [thread overview]
Message-ID: <56ACF885.8070203@redhat.com> (raw)
In-Reply-To: <20160130144403.GB32581@lerouge>
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.
--
All rights reversed
next prev parent reply other threads:[~2016-01-30 17:53 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-01-30 3:36 [PATCH 0/2] sched,time: reduce nohz_full syscall overhead 40% riel
2016-01-30 3:36 ` [PATCH 1/4] sched,time: remove non-power-of-two divides from __acct_update_integrals riel
2016-01-30 4:56 ` kbuild test robot
2016-01-30 14:44 ` Frederic Weisbecker
2016-01-30 17:53 ` Rik van Riel [this message]
2016-02-01 11:30 ` Peter Zijlstra
2016-01-30 3:36 ` [PATCH 2/4] acct,time: change indentation in __acct_update_integrals riel
2016-01-30 16:15 ` Frederic Weisbecker
2016-01-30 3:36 ` [PATCH 3/4] time,acct: drop irq save & restore from __acct_update_integrals riel
2016-01-30 16:24 ` Frederic Weisbecker
2016-01-30 3:36 ` [PATCH 4/4] sched,time: only call account_{user,sys,guest,idle}_time once a jiffy riel
2016-02-01 2:12 [PATCH 0/4 v3] sched,time: reduce nohz_full syscall overhead 40% riel
2016-02-01 2:12 ` [PATCH 1/4] sched,time: remove non-power-of-two divides from __acct_update_integrals riel
2016-02-01 4:46 ` kbuild test robot
2016-02-01 8:37 ` Thomas Gleixner
2016-02-01 9:22 ` Peter Zijlstra
2016-02-01 9:31 ` Thomas Gleixner
2016-02-01 13:44 ` Rik van Riel
2016-02-01 13:51 ` Peter Zijlstra
2016-02-01 19:21 [PATCH 0/4 v4] sched,time: reduce nohz_full syscall overhead 40% riel
2016-02-01 19:21 ` [PATCH 1/4] sched,time: remove non-power-of-two divides from __acct_update_integrals riel
2016-02-02 17:19 [PATCH 0/4 v5] sched,time: reduce nohz_full syscall overhead 40% riel
2016-02-02 17:19 ` [PATCH 1/4] sched,time: remove non-power-of-two divides from __acct_update_integrals riel
2016-02-11 1:08 [PATCH 0/4 v6] sched,time: reduce nohz_full syscall overhead 40% riel
2016-02-11 1:08 ` [PATCH 1/4] sched,time: remove non-power-of-two divides from __acct_update_integrals riel
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