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

[Marcelo Tosatti]

On Fri, Mar 04, 2022 at 04:35:54PM -0800, Andrew Morton wrote:
> On Fri, 4 Mar 2022 13:29:31 -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:
> > 
> > ...
> >
> > --- 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)) ||
> 
> Please changelog this alteration?

It should be now. Are you OK with this changelog ?
(if not, please let me know what should be improved).

On systems that run FIFO:1 applications that busy loop,
any SCHED_OTHER task that attempts to execute
on such a CPU (such as work threads) will not
be scheduled, which leads to system hangs.

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.

To fix this, replace the usage of work items with synchronize_rcu,
which provides the same guarantees:

Readers of lru_disable_count are protected by either disabling
preemption or rcu_read_lock:

preempt_disable, local_irq_disable  [bh_lru_lock()]
rcu_read_lock                       [rt_spin_lock CONFIG_PREEMPT_RT]
preempt_disable                     [local_lock !CONFIG_PREEMPT_RT]

Since v5.1 kernel, synchronize_rcu() is guaranteed to wait on
preempt_disable() regions of code. So any CPU which sees
lru_disable_count = 0 will have exited the critical
section when synchronize_rcu() returns.

Fixes:
...

Thanks.