sched/fair: Don't trigger active lb if src_rq->curr is not on_rq

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

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