Re: [PATCH v5 1/2] sched/fair: Don't trigger active lb if src_rq->curr is not on_rq

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jun 17, 2026 at 03:21:50PM +0800, Xin Zhao wrote:
> Active balancing needs the help by migration threads which will interrupt
> task on src_rq. It has a certain impact on overall performance. Active
> balancing often fails, there is a check to determine whether the current
> task(say it 'curr') on src_rq can run on dst_rq. We have observed that
> even that, if curr is a CFS task and on_rq is 0, the failure rate of
> active balancing is very high. Below are the test data from a certain
> fillback task scenario executed on a platform with 18 CPUs over 300
> seconds:

<snip table for brevity>

> In __schedule(), before setting curr to next, during the execution of
> pick_next_task(), sched_balance_rq() is called. It will unlock and then
> re-lock the rq, creating "holes" during which other CPUs may see zero
> rq->curr->on_rq. try_to_block_task() sets curr->on_rq to 0, and during the
> rq lock "hole" in pick_next_task(), rq->curr has not yet been assigned to
> next, resulting in curr->on_rq being seen as 0.
> 
> We do not need to perform active balancing when src_rq->curr is CFS task
> but on_rq is 0, as other CFS tasks have been probably checked just before.
> For cases where src_rq->curr is a non-CFS task, we retain the affinity
> check for dst_rq to trigger active balancing because such task is likely
> to wake-up or woken-by src_rq CFS task which has similar affinity
> characteristics to migrate. Also, after executing detach_tasks(), rq lock
> is released. Tasks on the rq awakened during detach_tasks() may preempt
> the previous CFS task. Based on my test(though not shown above), success
> rate of active balancing under the condition of !fair && on_rq is 98.4%.
> This scenario does not require the use of stop work, but need to add
> another path to detach attach task(s). It seems not necessary enough to
> add it, Valentin and Vincent have already discussed about it, see [1].
> 
> Additionally, sched_class field is a bit far from on_cpu in task_struct.
> The previous traversal of cfs_tasks checks on_cpu in can_migrate_task(),
> so the additional check for on_rq will not incur much cpu cycle loss, due
> to cache locality.
> 
> Two reasons why not check sched_class and on_rq of busiest->curr with the
> cpumask_test_cpu() check:
> 1. Let the PATCH not introduce new cases that skip logic for resetting
> balance_interval to min_interval.
> 2. The check of whether busiest cpu has been just triggered active balance
> filters a bit more cases than the check of sched_class and on_rq.
> 
> [1]: https://lore.kernel.org/lkml/20190815145107.5318-5-valentin.schneider@arm.com/
> 
> Signed-off-by: Xin Zhao <jackzxcui1989@163.com>

Perhaps because it is early and I've not had enough wake-up juice, or
perhaps because it is too damn warm already and my brain is
pre-emptively shutting down already, I found it very hard to read your
Changelog.

I asked one of these fancy AI things to rephrase things, and then did a
manual edit on it and ended up with the below. Notably, the changelog
talks about a sched_class check, while the actual patch has none of
that.

Does this work for you?

---
Subject: sched/fair: Don't trigger active lb if src_rq->curr is not on_rq
From: Xin Zhao <jackzxcui1989@163.com>
Date: Wed, 17 Jun 2026 15:21:50 +0800

From: Xin Zhao <jackzxcui1989@163.com>

Active load balancing relies on migration threads, which temporarily preempt
tasks on the source runqueue (src_rq). This preemption can negatively impact
overall system performance. The active balancing logic includes a check to
verify whether the current task (curr) on src_rq can actually run on the
destination runqueue (dst_rq). We have observed that when curr is a CFS task
and its on_rq flag is 0, the active balancing failure rate is exceptionally
high. The following table summarizes test data collected over 300 seconds on an
18-CPU platform under a specific fillback task scenario:

  fair: busiest->curr->sched_class == &fair_sched_class
  on_rq: busiest->curr->on_rq
  total: active balance count triggered of correspondent type
  fail: fail to migrate one task in active_load_balance_cpu_stop()

                   fair && !on_rq   !fair && !on_rq
         domain       total    fail   total    fail
  cpu0   0x00003          0       0       0       0
  cpu0   0x3ffff         33      33       1       1
  cpu1   0x00003          0       0       0       0
  cpu1   0x3ffff         42      42       0       0
  cpu2   0x0003c          4       4       0       0
  cpu2   0x3ffff         12      12       0       0
  cpu3   0x0003c          3       3       0       0
  cpu3   0x3ffff          8       7       0       0
  cpu4   0x0003c          2       2       0       0
  cpu4   0x3ffff          5       4       0       0
  cpu5   0x0003c          4       4       0       0
  cpu5   0x3ffff          8       8       0       0
  cpu6   0x003c0         60      60       0       0
  cpu6   0x3ffff         28      27       0       0
  cpu7   0x003c0        194     184       0       0
  cpu7   0x3ffff         35      35       1       1
  cpu8   0x003c0        240     228       0       0
  cpu8   0x3ffff         28      28       0       0
  cpu9   0x003c0          0       0       0       0
  cpu9   0x3ffff         10      10       0       0
  cpu10  0x03c00         52      50       0       0
  cpu10  0x3ffff          0       0       0       0
  cpu11  0x03c00         70      68       0       0
  cpu11  0x3ffff          1       1       0       0
  cpu12  0x03c00         73      72       0       0
  cpu12  0x3ffff          0       0       0       0
  cpu13  0x03c00         79      76       0       0
  cpu13  0x3ffff          0       0       0       0
  cpu14  0x3c000          0       0       0       0
  cpu14  0x3ffff         57      55       1       0
  cpu15  0x3c000         53      52       1       0
  cpu15  0x3ffff         30      29       0       0
  cpu16  0x3c000        344     341      10       6
  cpu16  0x3ffff        103     100       2       1
  cpu17  0x3c000        183     179       2       2
  cpu17  0x3ffff         78      77       0       0
  sum                  1839    1791      18      11

In __schedule(), before curr is updated to next, pick_next_task() invokes
sched_balance_rq(). This function temporarily unlocks and relocks the runqueue,
creating a window where other CPUs may observe rq->curr->on_rq as 0.

We can safely skip active balancing when src_rq->curr->on_rq == 0, as other
eligible tasks have likely already been evaluated. 

We retain the affinity check on dst_rq to trigger active balancing, since such
tasks are often woken by (or wake up) tasks on src_rq that share similar
affinity constraints. Furthermore, detach_tasks() releases the runqueue lock;
any tasks awakened during this window may preempt the previous CFS task. My
testing (data not shown) indicates that active balancing succeeds in 98.4% of
cases where !fair && on_rq.

This scenario does not require a stop-work callback, but would necessitate an
additional detach/attach path. As Valentin and Vincent have already discussed,
this addition does not appear justified at this time (see [1]).

Since can_migrate_task() already checks on_cpu during the cfs_tasks traversal,
adding an on_rq check will have negligible performance overhead due to cache
locality.

There are two reasons for not combining the on_rq check with the
cpumask_test_cpu() check:

 - Avoiding new scenarios that would skip the logic for resetting
   balance_interval to min_interval.

 - The existing check for whether the busiest CPU recently triggered active
   load balancing already filters more cases than the on_rq check.

[1]: https://lore.kernel.org/lkml/20190815145107.5318-5-valentin.schneider@arm.com/

Signed-off-by: Xin Zhao <jackzxcui1989@163.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Link: https://patch.msgid.link/20260617072151.1173416-2-jackzxcui1989@163.com
---
 kernel/sched/fair.c |   19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -13482,12 +13482,21 @@ static int sched_balance_rq(int this_cpu
 			 * ->active_balance_work.  Once set, it's cleared
 			 * only after active load balance is finished.
 			 */
-			if (!busiest->active_balance) {
-				busiest->active_balance = 1;
-				busiest->push_cpu = this_cpu;
-				active_balance = 1;
-			}
+			if (busiest->active_balance)
+				goto no_active_balance;
 
+			/*
+			 * @busiest dropped its rq_lock in the middle of
+			 * scheduling out its ->curr task (->on_rq := 0), no
+			 * need to forcefully punt it away with active balance.
+			 */
+			if (!busiest->curr->on_rq)
+				goto no_active_balance;
+
+			busiest->active_balance = 1;
+			busiest->push_cpu = this_cpu;
+			active_balance = 1;
+no_active_balance:
 			preempt_disable();
 			raw_spin_rq_unlock_irqrestore(busiest, flags);
 			if (active_balance) {