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

Neeraj Upadhyay
2024-06-06T03:46:21Z

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@amd.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@amd.com>
>>> Signed-off-by: Paul E. McKenney <paulmck@kernel.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? ;-)

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