[patch 0/2] replace work queue synchronization with synchronize_rcu

[Marcelo Tosatti <mtosatti@redhat.com>]

On systems that run FIFO:1 applications that busy loop 
on isolated CPUs, executing tasks on such CPUs under
lower priority is undesired (since that will either
hang the system, or cause longer interruption to the
FIFO task due to execution of lower priority task 
with very small sched slices).


Commit d479960e44f27e0e52ba31b21740b703c538027c ("mm: disable LRU 
pagevec during the migration temporarily") relies on 
queueing work items on all online CPUs to ensure visibility
of lru_disable_count.

However, its possible to use synchronize_rcu which will provide the same
guarantees:

    * synchronize_rcu() waits for preemption disabled
    * and RCU read side critical sections
    * For the users of lru_disable_count:
    *
    * preempt_disable, local_irq_disable() [bh_lru_lock()]
    * rcu_read_lock                        [lru_pvecs CONFIG_PREEMPT_RT]
    * preempt_disable                      [lru_pvecs !CONFIG_PREEMPT_RT]
    *
    *
    * so any calls of lru_cache_disabled wrapped by
    * local_lock+rcu_read_lock or preemption disabled would be
    * ordered by that. 


Fixes:

[ 1873.243925] INFO: task kworker/u160:0:9 blocked for more than 622 seconds.
[ 1873.243927]       Tainted: G          I      --------- ---  5.14.0-31.rt21.31.el9.x86_64 #1
[ 1873.243929] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 1873.243929] task:kworker/u160:0  state:D stack:    0 pid:    9 ppid:     2 flags:0x00004000
[ 1873.243932] Workqueue: cpuset_migrate_mm cpuset_migrate_mm_workfn
[ 1873.243936] Call Trace:
[ 1873.243938]  __schedule+0x21b/0x5b0
[ 1873.243941]  schedule+0x43/0xe0
[ 1873.243943]  schedule_timeout+0x14d/0x190
[ 1873.243946]  ? resched_curr+0x20/0xe0
[ 1873.243953]  ? __prepare_to_swait+0x4b/0x70
[ 1873.243958]  wait_for_completion+0x84/0xe0
[ 1873.243962]  __flush_work.isra.0+0x146/0x200
[ 1873.243966]  ? flush_workqueue_prep_pwqs+0x130/0x130
[ 1873.243971]  __lru_add_drain_all+0x158/0x1f0
[ 1873.243978]  do_migrate_pages+0x3d/0x2d0
[ 1873.243985]  ? pick_next_task_fair+0x39/0x3b0
[ 1873.243989]  ? put_prev_task_fair+0x1e/0x30
[ 1873.243992]  ? pick_next_task+0xb30/0xbd0
[ 1873.243995]  ? __tick_nohz_task_switch+0x1e/0x70
[ 1873.244000]  ? raw_spin_rq_unlock+0x18/0x60
[ 1873.244002]  ? finish_task_switch.isra.0+0xc1/0x2d0
[ 1873.244005]  ? __switch_to+0x12f/0x510
[ 1873.244013]  cpuset_migrate_mm_workfn+0x22/0x40
[ 1873.244016]  process_one_work+0x1e0/0x410
[ 1873.244019]  worker_thread+0x50/0x3b0
[ 1873.244022]  ? process_one_work+0x410/0x410
[ 1873.244024]  kthread+0x173/0x190
[ 1873.244027]  ? set_kthread_struct+0x40/0x40
[ 1873.244031]  ret_from_fork+0x1f/0x30 

diffstat:

 swap.c |  110 +++++++++++++++++++++++++++++++++++++++++++++++------------------
 1 file changed, 80 insertions(+), 30 deletions(-)