linux-kernel - Re: [PATCH v5 06/18] rcu: Introduce call_rcu

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Date:   Wed, 7 Sep 2022 09:44:01 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        rushikesh.s.kadam@...el.com, urezki@...il.com,
        neeraj.iitr10@...il.com, paulmck@...nel.org, rostedt@...dmis.org,
        vineeth@...byteword.org, boqun.feng@...il.com
Subject: Re: [PATCH v5 06/18] rcu: Introduce call_rcu_lazy() API
 implementation

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?

thanks,

 - Joel