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. Thanks Neeraj > > Thanx, Paul
