On Thu, Mar 10, 2022 at 10:22:12AM -0300, Marcelo Tosatti wrote: > > 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. > > Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx> > Reviewed-by: Nicolas Saenz Julienne <nsaenzju@xxxxxxxxxx> > Acked-by: Minchan Kim <minchan@xxxxxxxxxx> Someone pointed me at this: https://www.phoronix.com/scan.php?page=news_item&px=Linux-518-Stress-NUMA-Goes-Boom which says this one causes a performance regression with stress-ng's NUMA test... -- Regards/Gruss, Boris. https://people.kernel.org/tglx/notes-about-netiquette