Re: [patch v3] mm: lru_cache_disable: replace work queue synchronization with synchronize_rcu

[Paul E. McKenney]

On Thu, Mar 03, 2022 at 05:03:23PM -0800, Andrew Morton wrote:
> (Question for paulmck below, please)
> 
> On Tue, 22 Feb 2022 13:07:35 -0300 Marcelo Tosatti <mtosatti@xxxxxxxxxx> 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
> > 
> > ...
> >
> > --- a/mm/swap.c
> > +++ b/mm/swap.c
> > @@ -831,8 +831,7 @@ inline void __lru_add_drain_all(bool force_all_cpus)
> >  	for_each_online_cpu(cpu) {
> >  		struct work_struct *work = &per_cpu(lru_add_drain_work, cpu);
> >  
> > -		if (force_all_cpus ||
> > -		    pagevec_count(&per_cpu(lru_pvecs.lru_add, cpu)) ||
> > +		if (pagevec_count(&per_cpu(lru_pvecs.lru_add, cpu)) ||
> >  		    data_race(pagevec_count(&per_cpu(lru_rotate.pvec, cpu))) ||
> >  		    pagevec_count(&per_cpu(lru_pvecs.lru_deactivate_file, cpu)) ||
> >  		    pagevec_count(&per_cpu(lru_pvecs.lru_deactivate, cpu)) ||
> 
> This change appears to be "don't queue work on CPUs which don't have
> any work to do".  Correct?  This isn't changelogged?
> 
> > @@ -876,14 +875,19 @@ atomic_t lru_disable_count = ATOMIC_INIT(0);
> >  void lru_cache_disable(void)
> >  {
> >  	atomic_inc(&lru_disable_count);
> > +	synchronize_rcu();
> >  #ifdef CONFIG_SMP
> >  	/*
> > -	 * lru_add_drain_all in the force mode will schedule draining on
> > -	 * all online CPUs so any calls of lru_cache_disabled wrapped by
> > -	 * local_lock or preemption disabled would be ordered by that.
> > -	 * The atomic operation doesn't need to have stronger ordering
> > -	 * requirements because that is enforced by the scheduling
> > -	 * 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		       [rt_spin_lock CONFIG_PREEMPT_RT]
> > +	 * preempt_disable		       [local_lock !CONFIG_PREEMPT_RT]
> > +	 *
> > +	 * so any calls of lru_cache_disabled wrapped by local_lock or
> > +	 * preemption disabled would be ordered by that.
> >  	 */
> >  	__lru_add_drain_all(true);
> >  #else
> 
> Does this also work with CONFIG_TINY_RCU?
> 
> This seems abusive of synchronize_rcu().  None of this code uses RCU,
> but it so happens that synchronize_rcu() happily provides the desired
> effects.  Changes in RCU's happy side-effects might break this. 
> Perhaps a formal API function which does whatever-you-want-it-to-do
> would be better.

I don't claim to understand the full lru_cache_disable() use case, but
since v5.1 synchronize_rcu() is guaranteed to wait on preempt_disable()
regions of code.  In contrast, back in the old days, you had to use
synchronize_sched() to wait on preempt_disable() regions, even in
CONFIG_PREEMPT=y kernels.  So if the comment is accurate, it is OK.

Just be careful what you backport past v5.1...

> And...  I really don't understand the fix.  What is it about
> synchronize_rcu() which guarantees that a work function which is queued
> on CPU N will now get executed even if CPU N is spinning in SCHED_FIFO
> userspace?

I don't understand this part, either.

							Thanx, Paul