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

[Borislav Petkov]

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