Re: [PATCH v5 06/18] rcu: Introduce call_rcu_lazy() API implementation

[Joel Fernandes <joel@joelfernandes.org>]

On 9/7/2022 11:38 AM, Frederic Weisbecker wrote:
> On Wed, Sep 07, 2022 at 09:44:01AM -0400, Joel Fernandes wrote:
>> Hi Frederic,
>>
>> On 9/7/2022 5:40 AM, Frederic Weisbecker wrote:
>>>>
>>>> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
>>>> index bd8f39ee2cd0..e3344c262672 100644
>>>> --- a/kernel/rcu/tree_nocb.h
>>>> +++ b/kernel/rcu/tree_nocb.h
>>>> @@ -382,15 +382,19 @@ static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
>>>>  				  unsigned long j, unsigned long flush_flags)
>>>>  {
>>>>  	bool ret;
>>>> +	bool was_alldone;
>>>>  
>>>>  	if (!rcu_rdp_is_offloaded(rdp))
>>>>  		return true;
>>>>  	rcu_lockdep_assert_cblist_protected(rdp);
>>>>  	rcu_nocb_bypass_lock(rdp);
>>>> +	if (flush_flags & FLUSH_BP_WAKE)
>>>> +		was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
>>>> +
>>> You can check that outside bypass lock (but you still need nocb_lock).
>>
>> Right, ok. I can make it outside the bypass lock, and the nocb_lock is implied
>> by rcu_lockdep_assert_cblist_protected().
>>
>>>
>>>>  	ret = rcu_nocb_do_flush_bypass(rdp, rhp, j, flush_flags);
>>>>  
>>>> -	if (flush_flags & FLUSH_BP_WAKE)
>>>> -		wake_nocb_gp(rdp, true);
>>>> +	if (flush_flags & FLUSH_BP_WAKE && was_alldone)
>>>> +		wake_nocb_gp(rdp, false);
>>> That doesn't check if the bypass list was empty.
>>
>> thanks, will add your suggested optimization, however I have a general question:
>>
>> For the case where the bypass list is empty, where does rcu_barrier() do a wake
>> up of the nocb GP thread after entrain()?
>>
>> I don't see a call to __call_rcu_nocb_wake() like rcutree_migrate_callbacks()
>> does. Is the wake up done in some other path?
> 
> Actually I can't find it. You're right it may be missing. It's not even just
> about the bypass list but also the entrain'ed callback. Would you be willing to
> cook a fix?

Sounds good, yes I am happy to make a fix and include it in the next series
posting. Thanks for confirming the potential issue.

I will be on a plan in a few hours, I can even try to do it on the plane but
will see how it goes ;)

Thanks!

 - Joel