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Subject: Re: [PATCH] rcu: Report a quiescent state when it's exactly in the
 state
To:     Joel Fernandes <joel@joelfernandes.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc:     jiangshanlai@gmail.com, josh@joshtriplett.org, rostedt@goodmis.org,
        mathieu.desnoyers@efficios.com, linux-kernel@vger.kernel.org,
        kernel-team@lge.com, peterz@infradead.org
References: <1526027434-21237-1-git-send-email-byungchul.park@lge.com>
 <3af4cec0-4019-e3ac-77f9-8631252fb6da@lge.com>
 <20180511161746.GX26088@linux.vnet.ibm.com>
 <20180511224138.GA89902@joelaf.mtv.corp.google.com>
From:   Byungchul Park <byungchul.park@lge.com>
Message-ID: <9f2e445b-15b0-d1fa-832c-f801efc34d03@lge.com>
Date:   Mon, 14 May 2018 11:59:41 +0900
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On 2018-05-12 오전 7:41, Joel Fernandes wrote:
> On Fri, May 11, 2018 at 09:17:46AM -0700, Paul E. McKenney wrote:
>> On Fri, May 11, 2018 at 09:57:54PM +0900, Byungchul Park wrote:
>>> Hello folks,
>>>
>>> I think I wrote the title in a misleading way.
>>>
>>> Please change the title to something else such as,
>>> "rcu: Report a quiescent state when it's in the state" or,
>>> "rcu: Add points reporting quiescent states where proper" or so on.
>>>
>>> On 2018-05-11 오후 5:30, Byungchul Park wrote:
>>>> We expect a quiescent state of TASKS_RCU when cond_resched_tasks_rcu_qs()
>>>> is called, no matter whether it actually be scheduled or not. However,
>>>> it currently doesn't report the quiescent state when the task enters
>>>> into __schedule() as it's called with preempt = true. So make it report
>>>> the quiescent state unconditionally when cond_resched_tasks_rcu_qs() is
>>>> called.
>>>>
>>>> And in TINY_RCU, even though the quiescent state of rcu_bh also should
>>>> be reported when the tick interrupt comes from user, it doesn't. So make
>>>> it reported.
>>>>
>>>> Lastly in TREE_RCU, rcu_note_voluntary_context_switch() should be
>>>> reported when the tick interrupt comes from not only user but also idle,
>>>> as an extended quiescent state.
>>>>
>>>> Signed-off-by: Byungchul Park <byungchul.park@lge.com>
>>>> ---
>>>>   include/linux/rcupdate.h | 4 ++--
>>>>   kernel/rcu/tiny.c        | 6 +++---
>>>>   kernel/rcu/tree.c        | 4 ++--
>>>>   3 files changed, 7 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
>>>> index ee8cf5fc..7432261 100644
>>>> --- a/include/linux/rcupdate.h
>>>> +++ b/include/linux/rcupdate.h
>>>> @@ -195,8 +195,8 @@ static inline void exit_tasks_rcu_finish(void) { }
>>>>    */
>>>>   #define cond_resched_tasks_rcu_qs() \
>>>>   do { \
>>>> -	if (!cond_resched()) \
>>>> -		rcu_note_voluntary_context_switch_lite(current); \
>>>> +	rcu_note_voluntary_context_switch_lite(current); \
>>>> +	cond_resched(); \
>>
>> Ah, good point.
>>
>> Peter, I have to ask...  Why is "cond_resched()" considered a preemption
>> while "schedule()" is not?
> 
> Infact something interesting I inferred from the __schedule loop related to
> your question:
> 
> switch_count can either be set to prev->invcsw or prev->nvcsw. If we can
> assume that switch_count reflects whether the context switch is involuntary
> or voluntary,
>                        
> task-running-state	preempt		switch_count
> 0 (running)		1		involuntary
> 0			0		involuntary
> 1			0		voluntary
> 1			1		involuntary
> 
> According to the above table, both the task's running state and the preempt
> parameter to __schedule should be used together to determine if the switch is
> a voluntary one or not.
> 
> So this code in rcu_note_context_switch should really be:
> if (!preempt && !(current->state & TASK_RUNNING))
> 	rcu_note_voluntary_context_switch_lite(current);
> 
> According to the above table, cond_resched always classifies as an
> involuntary switch which makes sense to me. Even though cond_resched is

Hello guys,

The classification for nivcsw/nvcsw used in scheduler core, Joel, you
showed us is different from that used in when we distinguish between
non preemption/voluntary preemption/preemption/full and so on, even
they use the same word, "voluntary" though.

The name, rcu_note_voluntary_context_switch_lite() used in RCU has
a lot to do with the latter, the term of preemption. Furthermore, I
think the function should be called even when calling schedule() for
sleep as well. I think it would be better to change the function
name to something else to prevent confusing, it's up to Paul tho. :)

> explicitly called, its still sort of involuntary in the sense its not called
> into the scheduler for sleeping, but rather for seeing if something else can
> run instead (a preemption point). Infact none of the task deactivation in the
> __schedule loop will run if cond_resched is used.
> 
> I agree that if schedule was called directly but with TASK_RUNNING=1, then
> that could probably be classified an involuntary switch too...
> 
> Also since we're deciding to call rcu_note_voluntary_context_switch_lite
> unconditionally, then IMO this comment on that macro:
> 
> /*
>   * Note a voluntary context switch for RCU-tasks benefit.  This is a
>   * macro rather than an inline function to avoid #include hell.
>   */
>   #ifdef CONFIG_TASKS_RCU
>   #define rcu_note_voluntary_context_switch_lite(t)
> 
> Should be changed to:
> 
> /*
>   * Note a attempt to perform a voluntary context switch for RCU-tasks
>   * benefit.  This is called even in situations where a context switch
>   * didn't really happen even though it was requested. This is a
>   * macro rather than an inline function to avoid #include hell.
>   */
>   #ifdef CONFIG_TASKS_RCU
>   #define rcu_note_voluntary_context_switch_lite(t)
> 
> Right?
> 
> Correct me if I'm wrong about anything, thanks,
> 
> - Joel
> 
> 

-- 
Thanks,
Byungchul