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

[Paul E. McKenney]

On Thu, Jun 06, 2024 at 09:16:08AM +0530, Neeraj Upadhyay wrote:
> 
> 
> On 6/6/2024 12:08 AM, Paul E. McKenney wrote:
> > 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@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.
> > 
> > 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?  ;-)
> 
> Yes I agree that we postpone this change until we see it as a real
> problem. I had run a test to invoke synchronize_rcu() from all CPUs
> on a 96 core system in parallel. I didn't specifically check if this
> scenario was hit. Will run RCU torture test with this change.

Very well, I will drop this patch with the expectation that you will
re-post it if a problem does arise.

							Thanx, Paul