linux-kernel - Re: [PATCH] rcu: Remove the unnecessary separate function, rcu_preempt_do

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Date:   Mon, 26 Feb 2018 10:22:54 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Byungchul Park <byungchul.park@....com>, jiangshanlai@...il.com,
        josh@...htriplett.org, mathieu.desnoyers@...icios.com,
        linux-kernel@...r.kernel.org, kernel-team@....com
Subject: Re: [PATCH] rcu: Remove the unnecessary separate function,
 rcu_preempt_do_callback()

On Mon, Feb 26, 2018 at 12:15:14PM -0500, Steven Rostedt wrote:
> On Mon, 26 Feb 2018 14:11:36 +0900
> Byungchul Park <byungchul.park@....com> wrote:
> 
> > rcu_preemptp_do_callback() was introduced in commit 09223371dea(rcu:
> > Use softirq to address performance regression), where it had to be
> > distinguished between in the case CONFIG_TREE_PREEMPT_RCU is set and
> > it's not.
> > 
> > Now that the code was cleaned up so that rcu_preemt_do_callback() is
> > only called in rcu_kthread_do_work() in the same file, tree_plugin.h,
> > we don't have to keep the separate function anymore. Remove it for a
> > better readability.
> 
> Looks good to me (looks like commit f8b7fc6b51 "rcu: use softirq
> instead of kthreads except when RCU_BOOST=y" cleaned up the ifdefs and
> removed the requirement).
> 
> Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

Thank you both!  I have queued a slightly modified patch for testing
and further review.  Please see below and let me know if I messed
something up.

							Thanx, Paul

------------------------------------------------------------------------

commit b8a3012ddba397d4a18d9fd4a00432f8c2626bd6
Author: Byungchul Park <byungchul.park@....com>
Date:   Mon Feb 26 14:11:36 2018 +0900

    rcu: Inline rcu_preempt_do_callback() into its sole caller
    
    The rcu_preempt_do_callbacks() function was introduced in commit
    09223371dea(rcu: Use softirq to address performance regression), where it
    was necessary to handle kernel builds both containing and not containing
    RCU-preempt.  Since then, various changes (most notably f8b7fc6b51
    ("rcu: use softirq instead of kthreads except when RCU_BOOST=y")) have
    resulted in this function being invoked only from rcu_kthread_do_work(),
    which is present only in kernels containing RCU-preempt, which in turn
    means that the rcu_preempt_do_callbacks() function is no longer needed.
    
    This commit therefore inlines rcu_preempt_do_callbacks() into its
    sole remaining caller and also removes the rcu_state_p and rcu_data_p
    indirection for added clarity.
    
    Signed-off-by: Byungchul Park <byungchul.park@....com>
    Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
    [ paulmck: Remove the rcu_state_p and rcu_data_p indirection. ]

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index dc6f2319fc21..9dd0ea77faed 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -449,7 +449,6 @@ static void rcu_preempt_boost_start_gp(struct rcu_node *rnp);
 static void invoke_rcu_callbacks_kthread(void);
 static bool rcu_is_callbacks_kthread(void);
 #ifdef CONFIG_RCU_BOOST
-static void rcu_preempt_do_callbacks(void);
 static int rcu_spawn_one_boost_kthread(struct rcu_state *rsp,
 						 struct rcu_node *rnp);
 #endif /* #ifdef CONFIG_RCU_BOOST */
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 26d7a31e81cb..b0d7f9ba6bf2 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -686,15 +686,6 @@ static void rcu_preempt_check_callbacks(void)
 		t->rcu_read_unlock_special.b.need_qs = true;
 }
 
-#ifdef CONFIG_RCU_BOOST
-
-static void rcu_preempt_do_callbacks(void)
-{
-	rcu_do_batch(rcu_state_p, this_cpu_ptr(rcu_data_p));
-}
-
-#endif /* #ifdef CONFIG_RCU_BOOST */
-
 /**
  * call_rcu() - Queue an RCU callback for invocation after a grace period.
  * @head: structure to be used for queueing the RCU updates.
@@ -1170,7 +1161,7 @@ static void rcu_kthread_do_work(void)
 {
 	rcu_do_batch(&rcu_sched_state, this_cpu_ptr(&rcu_sched_data));
 	rcu_do_batch(&rcu_bh_state, this_cpu_ptr(&rcu_bh_data));
-	rcu_preempt_do_callbacks();
+	rcu_do_batch(&rcu_preempt_state, this_cpu_ptr(&rcu_preempt_data));
 }
 
 static void rcu_cpu_kthread_setup(unsigned int cpu)