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

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 10 Mar 2022 10:22:12 -0300 Marcelo Tosatti <mtosatti@redhat.com> 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.

Permitting a realtime thread to hang the entire system warrants a
-stable backport, I think.  That's just rude.

I'm inclined to send this upstream for 5.18-rc1, with that -stable tag.

But if agreeable, how far can we backport this?  Paul, do we know which
kernel version(s) have the desired synchronize_rcu() behaviour?

Now, we don't want -stable people backporting this into kernels where
synchronize_rcu() doesn't do what we want it to do.  So a sneaky thing
we could do is to identify the change which added the desired
synchronize_rcu() behaviour and make this patch Fixes:thatpatch.  That
should prevent people from backporting it too far.