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?
> 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();
