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

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 4 Mar 2022 13:29:31 -0300 Marcelo Tosatti <mtosatti@redhat.com> 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?

>  		    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)) ||
> @@ -876,15 +875,21 @@ atomic_t lru_disable_count = ATOMIC_INIT(0);
>  void lru_cache_disable(void)
>  {
>  	atomic_inc(&lru_disable_count);
> -#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.
> +	 * 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.
>  	 */
> +	synchronize_rcu();
> +#ifdef CONFIG_SMP
>  	__lru_add_drain_all(true);
>  #else
>  	lru_add_and_bh_lrus_drain();