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

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Date:   Wed, 7 Sep 2022 17:38:01 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.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

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?

Thanks!

> 
> thanks,
> 
>  - Joel