Re: [LKP] [lkp-developer] [sched/fair] 4e5160766f: +149% ftq.noise.50% regression

["Huang\, Ying" <ying.huang@intel.com>]

Vincent Guittot <vincent.guittot@linaro.org> writes:

> Hi Dietmar and Ying,
>
> Le Tuesday 03 Jan 2017  11:38:39 (+0100), Dietmar Eggemann a crit :
>> Hi Vincent and Ying,
>> 
>> On 01/02/2017 04:42 PM, Vincent Guittot wrote:
>> >Hi Ying,
>> >
>> >On 28 December 2016 at 09:17, Huang, Ying <ying.huang@intel.com> wrote:
>> >>Vincent Guittot <vincent.guittot@linaro.org> writes:
>> >>
>> >>>Le Tuesday 13 Dec 2016 . 09:47:30 (+0800), Huang, Ying a .crit :
>> >>>>Hi, Vincent,
>> >>>>
>> >>>>Vincent Guittot <vincent.guittot@linaro.org> writes:
>> 
>> [...]
>>
>
> [snip]
>
>> >>
>> >>The test result is as follow,
>> >>
>> >>=========================================================================================
>> >>compiler/cpufreq_governor/freq/kconfig/nr_task/rootfs/samples/tbox_group/test/testcase:
>> >>  gcc-6/powersave/20/x86_64-rhel-7.2/100%/debian-x86_64-2016-08-31.cgz/6000ss/lkp-hsw-d01/cache/ftq
>> >>
>> >>commit:
>> >>  4e5160766fcc9f41bbd38bac11f92dce993644aa: first bad commit
>> >>  09a43ace1f986b003c118fdf6ddf1fd685692d49: parent of first bad commit
>> >>  0613870ea53a7a279d8d37f2a3ce40aafc155fc8: debug commit with above patch
>> >>
>> >>4e5160766fcc9f41 09a43ace1f986b003c118fdf6d 0613870ea53a7a279d8d37f2a3
>> >>---------------- -------------------------- --------------------------
>> >>         %stddev     %change         %stddev     %change         %stddev
>> >>             \          |                \          |                \
>> >>     61670 228%     -96.5%       2148  11%     -94.7%       3281  58%  ftq.noise.25%
>> >>      3463  10%     -60.0%       1386  19%     -26.3%       2552  58%  ftq.noise.50%
>> >>      1116  23%     -72.6%     305.99  30%     -35.8%     716.15  64%  ftq.noise.75%
>> >>   3843815   3%      +3.1%    3963589   1%     -49.6%    1938221 100%  ftq.time.involuntary_context_switches
>> >>      5.33  30%     +21.4%       6.46  14%     -71.7%       1.50 108%  time.system_time
>> >>
>> >>
>> >>It appears that the system_time and involuntary_context_switches reduced
>> >>much after applied the debug patch, which is good from noise point of
>> >>view.  ftq.noise.50% reduced compared with the first bad commit, but
>> >>have not restored to that of the parent of the first bad commit.
>> >
>> >Thanks for testing. I will try to improve it a bit but not sure that I
>> >can reduce more.
>> 
>> Is this a desktop system where this regression comes from autogroups (1
>> level taskgroups) or a server system with systemd (2 level taskgroups)?
>> 
>> Since the PELT rewrite (v4.2) I have ~60 autogroups per cpu
>> (&rq->leaf_cfs_rq_list) on my Ubuntu desktop system permanently (Intel
>> i7-4750HQ) whereas in v4.1 there were 0 - 10.
>> 
>> $ for i in `seq 0 7`; do cat /proc/sched_debug | grep
>> "cfs_rq\[$i\]:/autogroup-" | wc -l; done
>> 58
>> 61
>> 63
>> 65
>> 60
>> 59
>> 62
>> 56
>> 
>> Couldn't we still remove these autogroups by if (!cfs_rq->nr_running &&
>> !se->avg.load_avg && !se->avg.util_avg) in update_blocked_averages()?
>> 
>> Vincent, like we discussed in September last year, the proper fix would
>> probably be a cfs-rq->nr_attached which IMHO is not doable w/o being an
>> atomic because of migrate_task_rq_fair()->remove_entity_load_avg() not
>> holding the rq lock.
>
> I remember the discussion and even if I agree that a large number of taskgroup
> increases the number of loop in update_blocked_averages() and as a result the
> time spent in the update, I don't think that this is the root cause of
> this regression because the patch "sched/fair: Propagate asynchrous detach"
> doesn't add more loops to update_blocked_averages but it adds more thing to do
> per loop.
>
> Then, I think I'm still too conservative in the condition for calling 
> update_load_avg(cfs_rq->tg->se[cpu], 0). This call has been added to
> propagate gcfs_rq->propagate_avg flag to parent so we don't need to call it 
> even if load_avg is not null but only when propagate_avg flag is set. The
> patch below should improve thing compare to the previous version because
> it will call update_load_avg(cfs_rq->tg->se[cpu], 0) only if an asynchrounous
> detach happened (propagate_avg is set).
>
> Ying, could you test the patch below instead of the previous one ?
>
> ---
>  kernel/sched/fair.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6559d19..a4f5c35 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6915,6 +6915,7 @@ static void update_blocked_averages(int cpu)
>  {
>  	struct rq *rq = cpu_rq(cpu);
>  	struct cfs_rq *cfs_rq;
> +	struct sched_entity *se;
>  	unsigned long flags;
>  
>  	raw_spin_lock_irqsave(&rq->lock, flags);
> @@ -6932,9 +6933,10 @@ static void update_blocked_averages(int cpu)
>  		if (update_cfs_rq_load_avg(cfs_rq_clock_task(cfs_rq), cfs_rq, true))
>  			update_tg_load_avg(cfs_rq, 0);
>  
> -		/* Propagate pending load changes to the parent */
> -		if (cfs_rq->tg->se[cpu])
> -			update_load_avg(cfs_rq->tg->se[cpu], 0);
> +		/* Propagate pending load changes to the parent if any */
> +		se = cfs_rq->tg->se[cpu];
> +		if (se && cfs_rq->propagate_avg)
> +			update_load_avg(se, 0);
>  	}
>  	raw_spin_unlock_irqrestore(&rq->lock, flags);
>  }

Here is the test result,

=========================================================================================
compiler/cpufreq_governor/freq/kconfig/nr_task/rootfs/samples/tbox_group/test/testcase:
  gcc-6/powersave/20/x86_64-rhel-7.2/100%/debian-x86_64-2016-08-31.cgz/6000ss/lkp-hsw-d01/cache/ftq

commit: 
  4e5160766fcc9f41bbd38bac11f92dce993644aa: first bad commit
  09a43ace1f986b003c118fdf6ddf1fd685692d49: parent of first bad commit
  b524060933c546fd2410c5a09360ba23a0fef846: with fix patch above

4e5160766fcc9f41 09a43ace1f986b003c118fdf6d b524060933c546fd2410c5a093 
---------------- -------------------------- -------------------------- 
         %stddev     %change         %stddev     %change         %stddev
             \          |                \          |                \  
      3463 ± 10%     -61.4%       1335 ± 17%      -3.0%       3359 ±  2%  ftq.noise.50%
      1116 ± 23%     -73.7%     293.90 ± 30%     -23.8%     850.69 ± 17%  ftq.noise.75%

Best Regards,
Huang, Ying