linux-kernel - Re: [PATCH 03/11] rcu: clean up rcu_preempt_deferred_qs

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Message-ID: <20191031185718.GY20975@paulmck-ThinkPad-P72>
Date:   Thu, 31 Oct 2019 11:57:18 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Lai Jiangshan <laijs@...ux.alibaba.com>
Cc:     linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org
Subject: Re: [PATCH 03/11] rcu: clean up rcu_preempt_deferred_qs_irqrestore()

On Thu, Oct 31, 2019 at 11:25:11PM +0800, Lai Jiangshan wrote:
> 
> 
> On 2019/10/31 9:52 下午, Paul E. McKenney wrote:
> > On Thu, Oct 31, 2019 at 10:07:58AM +0000, Lai Jiangshan wrote:
> > > Remove several unneeded return.
> > > 
> > > It doesn't need to return earlier after every code block.
> > > The code protects itself and be safe to fall through because
> > > every code block has its own condition tests.
> > > 
> > > Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
> > > ---
> > >   kernel/rcu/tree_plugin.h | 14 +-------------
> > >   1 file changed, 1 insertion(+), 13 deletions(-)
> > > 
> > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > > index 59ef10da1e39..82595db04eec 100644
> > > --- a/kernel/rcu/tree_plugin.h
> > > +++ b/kernel/rcu/tree_plugin.h
> > > @@ -439,19 +439,10 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
> > >   	 * t->rcu_read_unlock_special cannot change.
> > >   	 */
> > >   	special = t->rcu_read_unlock_special;
> > > -	rdp = this_cpu_ptr(&rcu_data);
> > > -	if (!special.s && !rdp->exp_deferred_qs) {
> > > -		local_irq_restore(flags);
> > > -		return;
> > > -	}
> > 
> > The point of this check is the common case of this function being invoked
> > when both fields are zero, avoiding the below redundant store and all the
> > extra checks of subfields of special.
> > 
> > Or are you saying that current compilers figure all this out?
> 
> No.
> 
> So, I have to keep the first/above return branch.
> 
> Any reasons to keep the following 2 return branches?
> There is no redundant store and the load for the checks
> are hot in the cache if the condition for return is met.

And the code further down is not in a fastpath.  So, good point, it
should be find to remove the two early exits below.

							Thanx, Paul

> Thanks.
> Lai
> 
> > 
> > 							Thanx, Paul
> > 
> > >   	t->rcu_read_unlock_special.b.deferred_qs = false;
> > >   	if (special.b.need_qs) {
> > >   		rcu_qs();
> > >   		t->rcu_read_unlock_special.b.need_qs = false;
> > > -		if (!t->rcu_read_unlock_special.s && !rdp->exp_deferred_qs) {
> > > -			local_irq_restore(flags);
> > > -			return;
> > > -		}
> > >   	}
> > >   	/*
> > > @@ -460,12 +451,9 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
> > >   	 * tasks are handled when removing the task from the
> > >   	 * blocked-tasks list below.
> > >   	 */
> > > +	rdp = this_cpu_ptr(&rcu_data);
> > >   	if (rdp->exp_deferred_qs) {
> > >   		rcu_report_exp_rdp(rdp);
> > > -		if (!t->rcu_read_unlock_special.s) {
> > > -			local_irq_restore(flags);
> > > -			return;
> > > -		}
> > >   	}
> > >   	/* Clean up if blocked during RCU read-side critical section. */
> > > -- 
> > > 2.20.1
> > >