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

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Date:   Tue, 27 Feb 2018 08:40:47 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     paulmck@...ux.vnet.ibm.com, Steven Rostedt <rostedt@...dmis.org>
Cc:     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 2/27/2018 8:35 AM, Byungchul Park wrote:
> On 2/27/2018 3:22 AM, Paul E. McKenney wrote:
>> 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));
> 
> OMG. Sorry for the mistake and thank you very much for fixing it.
> 
> I will be more careful.

Ah. Logically no difference between mine and your fixed one.

But anyway yours looks much better! Thank you~ :)

-- 
Thanks,
Byungchul