[RFC PATCH] sched: Simplify leaf_cfs_rq_list maintenance

[RFC PATCH] sched: Simplify leaf_cfs_rq_list maintenance

[Michal Koutný]

leaf_cfs_rq_list should contain cfs_rqs that have runnable entities in
them.  When we're operating under a throttled hierarchy we always update
the leaf_cfs_rq_list in order no to break list_add_leaf_cfs_rq invariant
of adding complete branches.

This caused troubles when an entity became runnable (enqueue_entity)
under a throttled hierarchy (see commit b34cb07dde7c ("sched/fair: Fix
enqueue_task_fair() warning some more")). While it proved well, this
new change ignores updating leaf_cfs_rq_list when we're operating under
the throttled hierarchy and defers the leaf_cfs_rq_list update to the
point when whole hiearchy is unthrottled (tg_unthrottle_up).

The code is now simpler and leaf_cfs_rq_list contains only cfs_rqs that
are truly runnable.

Why is this RFC?
- Primarily, I'm not sure I interpreted the purpose of leaf_cfs_rq_list
  right. The removal of throttled cfs_rqs from it would exclude them
  from __update_blocked_fair() calculation and I can't see past it now.
  If it's wrong assumption, I'd like this to help clarify what the
  proper definition of leaf_cfs_rq_list would be.
- Additionally, I didn't check thoroughly for corner cases when
  se->on_rq => cfs_rq_of(se)->on_list wouldn't hold, so the patch
  certainly isn't finished.

Signed-off-by: Michal Koutný <mkoutny@suse.com>
---
 kernel/sched/fair.c  | 41 ++++++-----------------------------------
 kernel/sched/sched.h |  4 +---
 2 files changed, 7 insertions(+), 38 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 04a3ce20da67..634ba6637824 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4250,10 +4250,11 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 
 	/*
 	 * When bandwidth control is enabled, cfs might have been removed
-	 * because of a parent been throttled but cfs->nr_running > 1. Try to
-	 * add it unconditionnally.
+	 * because of a parent been throttled. We'll add it later (with
+	 * complete branch up to se->on_rq/cfs_eq->on_list) in
+	 * tg_unthrottle_up() and unthrottle_cfs_rq().
 	 */
-	if (cfs_rq->nr_running == 1 || cfs_bandwidth_used())
+	if (cfs_rq->nr_running == 1 && !throttled_hierarchy(cfs_rq))
 		list_add_leaf_cfs_rq(cfs_rq);
 
 	if (cfs_rq->nr_running == 1)
@@ -4859,6 +4860,7 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
 	for_each_sched_entity(se) {
 		if (se->on_rq)
 			break;
+		// XXX: se->on_rq implies cfs_rq_of(se)->on_list (unless throttled_hierarchy)
 		cfs_rq = cfs_rq_of(se);
 		enqueue_entity(cfs_rq, se, ENQUEUE_WAKEUP);
 
@@ -4896,17 +4898,6 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
 	add_nr_running(rq, task_delta);
 
 unthrottle_throttle:
-	/*
-	 * The cfs_rq_throttled() breaks in the above iteration can result in
-	 * incomplete leaf list maintenance, resulting in triggering the
-	 * assertion below.
-	 */
-	for_each_sched_entity(se) {
-		cfs_rq = cfs_rq_of(se);
-
-		if (list_add_leaf_cfs_rq(cfs_rq))
-			break;
-	}
 
 	assert_list_leaf_cfs_rq(rq);
 
@@ -5518,6 +5509,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 	for_each_sched_entity(se) {
 		if (se->on_rq)
 			break;
+		// XXX: se->on_rq implies cfs_rq_of(se)->on_list (unless throttled_hierarchy)
 		cfs_rq = cfs_rq_of(se);
 		enqueue_entity(cfs_rq, se, flags);
 
@@ -5544,13 +5536,6 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 		/* end evaluation on encountering a throttled cfs_rq */
 		if (cfs_rq_throttled(cfs_rq))
 			goto enqueue_throttle;
-
-               /*
-                * One parent has been throttled and cfs_rq removed from the
-                * list. Add it back to not break the leaf list.
-                */
-               if (throttled_hierarchy(cfs_rq))
-                       list_add_leaf_cfs_rq(cfs_rq);
 	}
 
 	/* At this point se is NULL and we are at root level*/
@@ -5574,20 +5559,6 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 		update_overutilized_status(rq);
 
 enqueue_throttle:
-	if (cfs_bandwidth_used()) {
-		/*
-		 * When bandwidth control is enabled; the cfs_rq_throttled()
-		 * breaks in the above iteration can result in incomplete
-		 * leaf list maintenance, resulting in triggering the assertion
-		 * below.
-		 */
-		for_each_sched_entity(se) {
-			cfs_rq = cfs_rq_of(se);
-
-			if (list_add_leaf_cfs_rq(cfs_rq))
-				break;
-		}
-	}
 
 	assert_list_leaf_cfs_rq(rq);
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index bb09988451a0..f674d88920da 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -573,9 +573,7 @@ struct cfs_rq {
 	struct rq		*rq;	/* CPU runqueue to which this cfs_rq is attached */
 
 	/*
-	 * leaf cfs_rqs are those that hold tasks (lowest schedulable entity in
-	 * a hierarchy). Non-leaf lrqs hold other higher schedulable entities
-	 * (like users, containers etc.)
+	 * leaf cfs_rqs are those that hold runnable entities.
 	 *
 	 * leaf_cfs_rq_list ties together list of leaf cfs_rq's in a CPU.
 	 * This list is used during load balance.
-- 
2.30.1

Re: [RFC PATCH] sched: Simplify leaf_cfs_rq_list maintenance

[kernel test robot]

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Hi "Michal,

[FYI, it's a private test report for your RFC patch.]
[auto build test ERROR on tip/sched/core]
[also build test ERROR on v5.11 next-20210225]
[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/Michal-Koutn/sched-Simplify-leaf_cfs_rq_list-maintenance/20210226-003049
base:   https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git c5e6fc08feb2b88dc5dac2f3c817e1c2a4cafda4
config: sparc64-randconfig-s032-20210225 (attached as .config)
compiler: sparc64-linux-gcc (GCC) 9.3.0
reproduce:
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # apt-get install sparse
        # sparse version: v0.6.3-229-g60c1f270-dirty
        # https://github.com/0day-ci/linux/commit/e566cc27ca5c69d5199dd3b9ad07dfc4bdc35eac
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review Michal-Koutn/sched-Simplify-leaf_cfs_rq_list-maintenance/20210226-003049
        git checkout e566cc27ca5c69d5199dd3b9ad07dfc4bdc35eac
        # save the attached .config to linux build tree
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-9.3.0 make.cross C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__' ARCH=sparc64 

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 >>):

   In file included from arch/sparc/include/asm/bug.h:6,
                    from include/linux/bug.h:5,
                    from include/linux/thread_info.h:12,
                    from arch/sparc/include/asm/current.h:15,
                    from include/linux/sched.h:12,
                    from kernel/sched/sched.h:5,
                    from kernel/sched/fair.c:23:
   kernel/sched/fair.c: In function 'enqueue_entity':
>> kernel/sched/fair.c:4250:34: error: implicit declaration of function 'throttled_hierarchy' [-Werror=implicit-function-declaration]
    4250 |  if (cfs_rq->nr_running == 1 && !throttled_hierarchy(cfs_rq))
         |                                  ^~~~~~~~~~~~~~~~~~~
   include/linux/compiler.h:58:52: note: in definition of macro '__trace_if_var'
      58 | #define __trace_if_var(cond) (__builtin_constant_p(cond) ? (cond) : __trace_if_value(cond))
         |                                                    ^~~~
   kernel/sched/fair.c:4250:2: note: in expansion of macro 'if'
    4250 |  if (cfs_rq->nr_running == 1 && !throttled_hierarchy(cfs_rq))
         |  ^~
   kernel/sched/fair.c: At top level:
>> kernel/sched/fair.c:5361:19: error: static declaration of 'throttled_hierarchy' follows non-static declaration
    5361 | static inline int throttled_hierarchy(struct cfs_rq *cfs_rq)
         |                   ^~~~~~~~~~~~~~~~~~~
   In file included from arch/sparc/include/asm/bug.h:6,
                    from include/linux/bug.h:5,
                    from include/linux/thread_info.h:12,
                    from arch/sparc/include/asm/current.h:15,
                    from include/linux/sched.h:12,
                    from kernel/sched/sched.h:5,
                    from kernel/sched/fair.c:23:
   kernel/sched/fair.c:4250:34: note: previous implicit declaration of 'throttled_hierarchy' was here
    4250 |  if (cfs_rq->nr_running == 1 && !throttled_hierarchy(cfs_rq))
         |                                  ^~~~~~~~~~~~~~~~~~~
   include/linux/compiler.h:58:52: note: in definition of macro '__trace_if_var'
      58 | #define __trace_if_var(cond) (__builtin_constant_p(cond) ? (cond) : __trace_if_value(cond))
         |                                                    ^~~~
   kernel/sched/fair.c:4250:2: note: in expansion of macro 'if'
    4250 |  if (cfs_rq->nr_running == 1 && !throttled_hierarchy(cfs_rq))
         |  ^~
   kernel/sched/fair.c:5372:6: warning: no previous prototype for 'init_cfs_bandwidth' [-Wmissing-prototypes]
    5372 | void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b) {}
         |      ^~~~~~~~~~~~~~~~~~
   kernel/sched/fair.c:11160:6: warning: no previous prototype for 'free_fair_sched_group' [-Wmissing-prototypes]
   11160 | void free_fair_sched_group(struct task_group *tg) { }
         |      ^~~~~~~~~~~~~~~~~~~~~
   kernel/sched/fair.c:11162:5: warning: no previous prototype for 'alloc_fair_sched_group' [-Wmissing-prototypes]
   11162 | int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
         |     ^~~~~~~~~~~~~~~~~~~~~~
   kernel/sched/fair.c:11167:6: warning: no previous prototype for 'online_fair_sched_group' [-Wmissing-prototypes]
   11167 | void online_fair_sched_group(struct task_group *tg) { }
         |      ^~~~~~~~~~~~~~~~~~~~~~~
   kernel/sched/fair.c:11169:6: warning: no previous prototype for 'unregister_fair_sched_group' [-Wmissing-prototypes]
   11169 | void unregister_fair_sched_group(struct task_group *tg) { }
         |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~
   cc1: some warnings being treated as errors

Kconfig warnings: (for reference only)
   WARNING: unmet direct dependencies detected for FRAME_POINTER
   Depends on DEBUG_KERNEL && (M68K || UML || SUPERH) || ARCH_WANT_FRAME_POINTERS || MCOUNT
   Selected by
   - LOCKDEP && DEBUG_KERNEL && LOCK_DEBUGGING_SUPPORT && !MIPS && !PPC && !ARM && !S390 && !MICROBLAZE && !ARC && !X86


vim +/throttled_hierarchy +4250 kernel/sched/fair.c

  4166	
  4167	/*
  4168	 * MIGRATION
  4169	 *
  4170	 *	dequeue
  4171	 *	  update_curr()
  4172	 *	    update_min_vruntime()
  4173	 *	  vruntime -= min_vruntime
  4174	 *
  4175	 *	enqueue
  4176	 *	  update_curr()
  4177	 *	    update_min_vruntime()
  4178	 *	  vruntime += min_vruntime
  4179	 *
  4180	 * this way the vruntime transition between RQs is done when both
  4181	 * min_vruntime are up-to-date.
  4182	 *
  4183	 * WAKEUP (remote)
  4184	 *
  4185	 *	->migrate_task_rq_fair() (p->state == TASK_WAKING)
  4186	 *	  vruntime -= min_vruntime
  4187	 *
  4188	 *	enqueue
  4189	 *	  update_curr()
  4190	 *	    update_min_vruntime()
  4191	 *	  vruntime += min_vruntime
  4192	 *
  4193	 * this way we don't have the most up-to-date min_vruntime on the originating
  4194	 * CPU and an up-to-date min_vruntime on the destination CPU.
  4195	 */
  4196	
  4197	static void
  4198	enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
  4199	{
  4200		bool renorm = !(flags & ENQUEUE_WAKEUP) || (flags & ENQUEUE_MIGRATED);
  4201		bool curr = cfs_rq->curr == se;
  4202	
  4203		/*
  4204		 * If we're the current task, we must renormalise before calling
  4205		 * update_curr().
  4206		 */
  4207		if (renorm && curr)
  4208			se->vruntime += cfs_rq->min_vruntime;
  4209	
  4210		update_curr(cfs_rq);
  4211	
  4212		/*
  4213		 * Otherwise, renormalise after, such that we're placed at the current
  4214		 * moment in time, instead of some random moment in the past. Being
  4215		 * placed in the past could significantly boost this task to the
  4216		 * fairness detriment of existing tasks.
  4217		 */
  4218		if (renorm && !curr)
  4219			se->vruntime += cfs_rq->min_vruntime;
  4220	
  4221		/*
  4222		 * When enqueuing a sched_entity, we must:
  4223		 *   - Update loads to have both entity and cfs_rq synced with now.
  4224		 *   - Add its load to cfs_rq->runnable_avg
  4225		 *   - For group_entity, update its weight to reflect the new share of
  4226		 *     its group cfs_rq
  4227		 *   - Add its new weight to cfs_rq->load.weight
  4228		 */
  4229		update_load_avg(cfs_rq, se, UPDATE_TG | DO_ATTACH);
  4230		se_update_runnable(se);
  4231		update_cfs_group(se);
  4232		account_entity_enqueue(cfs_rq, se);
  4233	
  4234		if (flags & ENQUEUE_WAKEUP)
  4235			place_entity(cfs_rq, se, 0);
  4236	
  4237		check_schedstat_required();
  4238		update_stats_enqueue(cfs_rq, se, flags);
  4239		check_spread(cfs_rq, se);
  4240		if (!curr)
  4241			__enqueue_entity(cfs_rq, se);
  4242		se->on_rq = 1;
  4243	
  4244		/*
  4245		 * When bandwidth control is enabled, cfs might have been removed
  4246		 * because of a parent been throttled. We'll add it later (with
  4247		 * complete branch up to se->on_rq/cfs_eq->on_list) in
  4248		 * tg_unthrottle_up() and unthrottle_cfs_rq().
  4249		 */
> 4250		if (cfs_rq->nr_running == 1 && !throttled_hierarchy(cfs_rq))
  4251			list_add_leaf_cfs_rq(cfs_rq);
  4252	
  4253		if (cfs_rq->nr_running == 1)
  4254			check_enqueue_throttle(cfs_rq);
  4255	}
  4256	

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

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Re: [RFC PATCH] sched: Simplify leaf_cfs_rq_list maintenance

[Dietmar Eggemann]

Hi Michal,

On 25/02/2021 17:27, Michal Koutný wrote:
> leaf_cfs_rq_list should contain cfs_rqs that have runnable entities in
> them.  When we're operating under a throttled hierarchy we always update
> the leaf_cfs_rq_list in order no to break list_add_leaf_cfs_rq invariant
> of adding complete branches.

So this patch replaces in enqueue_entity() the unconditional
list_add_leaf_cfs_rq(cfs_rq) in case cfs->nr_running > 1
(parent had been throttled) 

- if (cfs_rq->nr_running == 1 || cfs_bandwidth_used())
                              ^^^^^^^^^^^^^^^^^^^^^^^
with

+ if (cfs_rq->nr_running == 1 && !throttled_hierarchy(cfs_rq))
                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

and removes the leaf_cfs_rq_list maintenance code after the
xxx_throttle label in enqueue_task_fair() and unthrottle_cfs_rq
from commit f6783319737f ("sched/fair: Fix insertion in
rq->leaf_cfs_rq_list") and fe61468b2cb ("sched/fair: Fix
enqueue_task_fair warning").

> This caused troubles when an entity became runnable (enqueue_entity)
> under a throttled hierarchy (see commit b34cb07dde7c ("sched/fair: Fix
> enqueue_task_fair() warning some more")). While it proved well, this
> new change ignores updating leaf_cfs_rq_list when we're operating under
> the throttled hierarchy and defers the leaf_cfs_rq_list update to the
> point when whole hiearchy is unthrottled (tg_unthrottle_up).

IMHO, f6783319737f gives the explanation why throttled cfs_rq's have to
be added to rq->leaf_cfs_rq_list.

IIRC, fe61468b2cb was fixing a use case in which a cfs-rq with
on_list=0 and nr_running > 1 within the cgroup hierarchy wasn't
added back to rq->leaf_cfs_rq_list:

https://lkml.kernel.org/r/15252de5-9a2d-19ae-607a-594ee88d1ba1@de.ibm.com

In enqueue_task_fair() just before the assert_list_leaf_cfs_rq(rq),
Iterate through the se heriarchy of p=[CPU 2/KVM 2662] in case the
assert will hit:

...
CPU23 path=/machine.slice/machine-test.slice/machine-qemu\x2d18\x2dtest10.scope/vcpuX on_list=1 nr_running=1 throttled=0 p=[CPU 2/KVM 2662]
CPU23 path=/machine.slice/machine-test.slice/machine-qemu\x2d18\x2dtest10.scope       on_list=0 nr_running=3 throttled=0 p=[CPU 2/KVM 2662]
                                                                                      ^^^^^^^^^ ^^^^^^^^^^^^
CPU23 path=/machine.slice/machine-test.slice                                          on_list=1 nr_running=1 throttled=1 p=[CPU 2/KVM 2662]
CPU23 path=/machine.slice                                                             on_list=1 nr_running=0 throttled=0 p=[CPU 2/KVM 2662]
CPU23 path=/                                                                          on_list=1 nr_running=1 throttled=0 p=[CPU 2/KVM 2662]
...

> The code is now simpler and leaf_cfs_rq_list contains only cfs_rqs that
> are truly runnable.
> 
> Why is this RFC?
> - Primarily, I'm not sure I interpreted the purpose of leaf_cfs_rq_list
>   right. The removal of throttled cfs_rqs from it would exclude them
>   from __update_blocked_fair() calculation and I can't see past it now.

The leaf_cfs_rq_list should contain all cfs_rqs with
cfs_rq->avg.load/runnable/util_avg != 0 so that in case there are no
runnable entities on them anymore this (blocked) load 
cfs_rq->avg.xxx_avg can be decayed. IMHO. the "leaf_" is misleading
here since it can also contain non-leaf cfs_rqs.

>   If it's wrong assumption, I'd like this to help clarify what the
>   proper definition of leaf_cfs_rq_list would be.
> - Additionally, I didn't check thoroughly for corner cases when
>   se->on_rq => cfs_rq_of(se)->on_list wouldn't hold, so the patch
>   certainly isn't finished.

[...]