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

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Date: Wed, 5 Jun 2024 11:38:50 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Frederic Weisbecker <frederic@...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

On Wed, Jun 05, 2024 at 02:09:34PM +0200, Frederic Weisbecker wrote:
> 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.

True, which is why we agreed not to submit it into the v6.10 merge window.

And I don't recall us saying what merge window to send it to.

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

Neeraj, thoughts?  Or, better yet, test results?  ;-)

							Thanx, Paul