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Date:   Thu, 31 Mar 2022 15:52:45 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Marcelo Tosatti <mtosatti@redhat.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Minchan Kim <minchan@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        Nicolas Saenz Julienne <nsaenzju@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        "Paul E. McKenney" <paulmck@kernel.org>
Subject: Re: [patch v5] mm: lru_cache_disable: replace work queue
 synchronization with synchronize_rcu
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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...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette