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

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Message-ID: <ZmBVfpyEZeTIAHJn@localhost.localdomain>
Date: Wed, 5 Jun 2024 14:09:34 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
	rostedt@...dmis.org, Neeraj Upadhyay <Neeraj.Upadhyay@....com>
Subject: Re: [PATCH rcu 2/9] rcu: Reduce synchronize_rcu() delays when all
 wait heads are in use

Le Tue, Jun 04, 2024 at 03:23:48PM -0700, Paul E. McKenney a écrit :
> From: Neeraj Upadhyay <Neeraj.Upadhyay@....com>
> 
> 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@....com>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>

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.