Re: [patch v4] mm: lru_cache_disable: replace work queue synchronization with synchronize_rcu

[Minchan Kim <minchan@kernel.org>]

On Fri, Mar 04, 2022 at 01:29:31PM -0300, Marcelo Tosatti wrote:
>  
> 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 (see comment this patch modifies on lru_cache_disable).
> 
> 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
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

Looks like great to me. 

Acked-by: Minchan Kim <minchan@kernel.org>

While I reviewed it, it seems I found a bug that br_lru_install
needs to check lru_cache_disabled under bh_lru_lock. Let me
cook the patch.

Thanks!