Re: [PATCH rcu 2/9] rcu: Reduce synchronize_rcu() delays when all wait heads are in use

[Frederic Weisbecker]

Le Tue, Jun 04, 2024 at 03:23:48PM -0700, Paul E. McKenney a écrit :
> From: Neeraj Upadhyay <Neeraj.Upadhyay@xxxxxxx>
> 
> When all wait heads are in use, which can happen when
> rcu_sr_normal_gp_cleanup_work()'s callback processing
> is slow, any new synchronize_rcu() user's rcu_synchronize
> node's processing is deferred to future GP periods. This
> can result in long list of synchronize_rcu() invocations
> waiting for full grace period processing, which can delay
> freeing of memory. Mitigate this problem by using first
> node in the list as wait tail when all wait heads are in use.
> While methods to speed up callback processing would be needed
> to recover from this situation, allowing new nodes to complete
> their grace period can help prevent delays due to a fixed
> number of wait head nodes.
> 
> Signed-off-by: Neeraj Upadhyay <Neeraj.Upadhyay@xxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

IIRC we agreed that this patch could be a step too far that
made an already not so simple state machine even less simple,
breaking the wait_head based flow.

Should we postpone this change until it is observed that a workqueue
not being scheduled for 5 grace periods is a real issue?

Thanks.