Re: [patch v5] mm: lru_cache_disable: replace work queue synchronization with synchronize_rcu

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Mar 31, 2022 at 03:52:45PM +0200, Borislav Petkov wrote:
> 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@redhat.com>
> > Reviewed-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
> > Acked-by: Minchan Kim <minchan@kernel.org>
> 
> 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...

Michael,

This is probably do_migrate_pages that is taking too long due to
synchronize_rcu().

Switching to synchronize_rcu_expedited() should probably fix it...
Can you give it a try, please?

diff --git a/mm/swap.c b/mm/swap.c
index bceff0cb559c..04a8bbf9817a 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -879,7 +879,7 @@ void lru_cache_disable(void)
 	 * lru_disable_count = 0 will have exited the critical
 	 * section when synchronize_rcu() returns.
 	 */
-	synchronize_rcu();
+	synchronize_rcu_expedited();
 #ifdef CONFIG_SMP
 	__lru_add_drain_all(true);
 #else