[PATCH v2] sched/fair: Correctly insert cfs_rq's to list on unthrottle

[PATCH v2] sched/fair: Correctly insert cfs_rq's to list on unthrottle

[Odin Ugedal]

This fixes an issue where fairness is decreased since cfs_rq's can
end up not being decayed properly. For two sibling control groups with
the same priority, this can often lead to a load ratio of 99/1 (!!).

This happen because when a cfs_rq is throttled, all the descendant cfs_rq's
will be removed from the leaf list. When they initial cfs_rq is
unthrottled, it will currently only re add descendant cfs_rq's if they
have one or more entities enqueued. This is not a perfect heuristic.

Insted, we insert all cfs_rq's that contain one or more enqueued
entities, or contributes to the load of the task group.

Can often lead to sutiations like this for equally weighted control
groups:

$ ps u -C stress
USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root       10009 88.8  0.0   3676   100 pts/1    R+   11:04   0:13 stress --cpu 1
root       10023  3.0  0.0   3676   104 pts/1    R+   11:04   0:00 stress --cpu 1

Fixes: 31bc6aeaab1d ("sched/fair: Optimize update_blocked_averages()")
Signed-off-by: Odin Ugedal <odin@uged.al>
---

Original thread: https://lore.kernel.org/lkml/20210518125202.78658-3-odin@uged.al/
Changes since v1:
 - Replaced cfs_rq field with using tg_load_avg_contrib
 - Went from 3 to 1 pathces; one is merged and one is replaced
   by a new patchset.

 kernel/sched/fair.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 794c2cb945f8..0f1b39ca5ca8 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4719,8 +4719,11 @@ static int tg_unthrottle_up(struct task_group *tg, void *data)
 		cfs_rq->throttled_clock_task_time += rq_clock_task(rq) -
 					     cfs_rq->throttled_clock_task;
 
-		/* Add cfs_rq with already running entity in the list */
-		if (cfs_rq->nr_running >= 1)
+		/*
+		 * Add cfs_rq with tg load avg contribution or one or more
+		 * already running entities to the list
+		 */
+		if (cfs_rq->tg_load_avg_contrib || cfs_rq->nr_running)
 			list_add_leaf_cfs_rq(cfs_rq);
 	}
 
-- 
2.31.1

Re: [PATCH v2] sched/fair: Correctly insert cfs_rq's to list on unthrottle

[Vincent Guittot]

On Thu, 3 Jun 2021 at 13:41, Odin Ugedal <odin@uged.al> wrote:
>
> This fixes an issue where fairness is decreased since cfs_rq's can
> end up not being decayed properly. For two sibling control groups with
> the same priority, this can often lead to a load ratio of 99/1 (!!).
>
> This happen because when a cfs_rq is throttled, all the descendant cfs_rq's
> will be removed from the leaf list. When they initial cfs_rq is
> unthrottled, it will currently only re add descendant cfs_rq's if they
> have one or more entities enqueued. This is not a perfect heuristic.
>
> Insted, we insert all cfs_rq's that contain one or more enqueued
> entities, or contributes to the load of the task group.
>
> Can often lead to sutiations like this for equally weighted control
> groups:
>
> $ ps u -C stress
> USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
> root       10009 88.8  0.0   3676   100 pts/1    R+   11:04   0:13 stress --cpu 1
> root       10023  3.0  0.0   3676   104 pts/1    R+   11:04   0:00 stress --cpu 1
>
> Fixes: 31bc6aeaab1d ("sched/fair: Optimize update_blocked_averages()")
> Signed-off-by: Odin Ugedal <odin@uged.al>
> ---
>
> Original thread: https://lore.kernel.org/lkml/20210518125202.78658-3-odin@uged.al/
> Changes since v1:
>  - Replaced cfs_rq field with using tg_load_avg_contrib
>  - Went from 3 to 1 pathces; one is merged and one is replaced
>    by a new patchset.
>
>  kernel/sched/fair.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 794c2cb945f8..0f1b39ca5ca8 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4719,8 +4719,11 @@ static int tg_unthrottle_up(struct task_group *tg, void *data)
>                 cfs_rq->throttled_clock_task_time += rq_clock_task(rq) -
>                                              cfs_rq->throttled_clock_task;
>
> -               /* Add cfs_rq with already running entity in the list */
> -               if (cfs_rq->nr_running >= 1)
> +               /*
> +                * Add cfs_rq with tg load avg contribution or one or more
> +                * already running entities to the list
> +                */
> +               if (cfs_rq->tg_load_avg_contrib || cfs_rq->nr_running)

Out of curiosity, why did you decide to use
cfs_rq->tg_load_avg_contrib instead of !cfs_rq_is_decayed(cfs_rq)
which is used to delete the cfs_rq from the list when updating blocked
load ?

>                         list_add_leaf_cfs_rq(cfs_rq);
>         }
>
> --
> 2.31.1
>

Re: [PATCH v2] sched/fair: Correctly insert cfs_rq's to list on unthrottle

[Odin Ugedal]

Hi,

> Out of curiosity, why did you decide to use
> cfs_rq->tg_load_avg_contrib instead of !cfs_rq_is_decayed(cfs_rq)
> which is used to delete the cfs_rq from the list when updating blocked
> load ?

Well, the main reason was that it is currently (without the other in
flight patches) not safe to just use "cfs_rq_is_decayed" directly,
since that could result in
a situation where tg_load_avg_contrib!=0 while
cfs_rq_is_decayed()==true. I guess we can use cfs_rq_is_decayed() if
you prefer that,
and all the other PELT patches are merged. (This was initially why I
thought a new field was a simpler and more elegant solution to make
sure we book-keep correctly,
but when the PELT stuff is fixed properly, that should be no real
issue as long it works as we expect).

I was also thinking about the cfs_rq->nr_running part; is there a
chance of a situation where a cfs_rq->nr_running==1 and it has no
load, resulting in it being decayed and
removed from the list in "__update_blocked_fair"? I have not looked
properly at it, but just wondering if that is actually possible..


Also, out of curiosity, are there some implications of a situation
where tg_load_avg_contrib=0 while *_load!=0, or would that not cause
fairness issues?

Thanks
Odin

Re: [PATCH v2] sched/fair: Correctly insert cfs_rq's to list on unthrottle

[Vincent Guittot]

On Thu, 3 Jun 2021 at 15:13, Odin Ugedal <odin@uged.al> wrote:
>
> Hi,
>
> > Out of curiosity, why did you decide to use
> > cfs_rq->tg_load_avg_contrib instead of !cfs_rq_is_decayed(cfs_rq)
> > which is used to delete the cfs_rq from the list when updating blocked
> > load ?
>
> Well, the main reason was that it is currently (without the other in
> flight patches) not safe to just use "cfs_rq_is_decayed" directly,
> since that could result in
> a situation where tg_load_avg_contrib!=0 while
> cfs_rq_is_decayed()==true. I guess we can use cfs_rq_is_decayed() if
> you prefer that,
> and all the other PELT patches are merged. (This was initially why I
> thought a new field was a simpler and more elegant solution to make
> sure we book-keep correctly,
> but when the PELT stuff is fixed properly, that should be no real
> issue as long it works as we expect).

If it's only a matter of waiting other PELT patches to be merged, we
should use cfs_rq_is_decayed().

cfs_rq->tg_load_avg_contrib is used to reduce contention on
tg->load_avg but it is outside the scope of PELT and the update of
blocked load so we should avoid using it there

>
> I was also thinking about the cfs_rq->nr_running part; is there a
> chance of a situation where a cfs_rq->nr_running==1 and it has no
> load, resulting in it being decayed and
> removed from the list in "__update_blocked_fair"? I have not looked
> properly at it, but just wondering if that is actually possible..
>
>
> Also, out of curiosity, are there some implications of a situation
> where tg_load_avg_contrib=0 while *_load!=0, or would that not cause

do you mean all cfs_rq->avg.load_avg with *_load ?

> fairness issues?

if load_avg!=0, we will update it periodically and sync
tg_load_avg_contrib with the former. So it's not a problem.

The other way was a problem because we stop updating load_avg and
tg_load_avg_contrib when load_avg/load_sum is null so the
tg_load_avg_contrib is stalled with a possibly very old value


>
> Thanks
> Odin

Re: [PATCH v2] sched/fair: Correctly insert cfs_rq's to list on unthrottle

[Odin Ugedal]

> If it's only a matter of waiting other PELT patches to be merged, we
> should use cfs_rq_is_decayed().

ACK. will post a v3.

> if load_avg!=0, we will update it periodically and sync
> tg_load_avg_contrib with the former. So it's not a problem.
>
> The other way was a problem because we stop updating load_avg and
> tg_load_avg_contrib when load_avg/load_sum is null so the
> tg_load_avg_contrib is stalled with a possibly very old value

Yeah, that makes sense.

Thanks
Odin

Re: [PATCH v2] sched/fair: Correctly insert cfs_rq's to list on unthrottle

[kernel test robot]

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Hi Odin,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on tip/sched/core]
[also build test ERROR on tip/master linux/master linus/master v5.13-rc4 next-20210603]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/0day-ci/linux/commits/Odin-Ugedal/sched-fair-Correctly-insert-cfs_rq-s-to-list-on-unthrottle/20210603-194311
base:   https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git 858f9e11be8855ed62cb97e58174515da595c76b
config: h8300-randconfig-r001-20210603 (attached as .config)
compiler: h8300-linux-gcc (GCC) 9.3.0
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # https://github.com/0day-ci/linux/commit/fd98f41f84acfad0ad1c7d1eb553807e8d1e0b9b
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review Odin-Ugedal/sched-fair-Correctly-insert-cfs_rq-s-to-list-on-unthrottle/20210603-194311
        git checkout fd98f41f84acfad0ad1c7d1eb553807e8d1e0b9b
        # save the attached .config to linux build tree
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-9.3.0 make.cross ARCH=h8300 

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <lkp@intel.com>

All errors (new ones prefixed by >>):

   kernel/sched/fair.c: In function 'tg_unthrottle_up':
>> kernel/sched/fair.c:4704:13: error: 'struct cfs_rq' has no member named 'tg_load_avg_contrib'
    4704 |   if (cfs_rq->tg_load_avg_contrib || cfs_rq->nr_running)
         |             ^~


vim +4704 kernel/sched/fair.c

  4689	
  4690	static int tg_unthrottle_up(struct task_group *tg, void *data)
  4691	{
  4692		struct rq *rq = data;
  4693		struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
  4694	
  4695		cfs_rq->throttle_count--;
  4696		if (!cfs_rq->throttle_count) {
  4697			cfs_rq->throttled_clock_task_time += rq_clock_task(rq) -
  4698						     cfs_rq->throttled_clock_task;
  4699	
  4700			/*
  4701			 * Add cfs_rq with tg load avg contribution or one or more
  4702			 * already running entities to the list
  4703			 */
> 4704			if (cfs_rq->tg_load_avg_contrib || cfs_rq->nr_running)
  4705				list_add_leaf_cfs_rq(cfs_rq);
  4706		}
  4707	
  4708		return 0;
  4709	}
  4710	

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all(a)lists.01.org

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