linux-kernel - Re: [PATCH] rcu: Report a quiescent state when it's exactly in the state

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Date:   Tue, 15 May 2018 09:18:57 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     paulmck@...ux.vnet.ibm.com
Cc:     Joel Fernandes <joel@...lfernandes.org>, jiangshanlai@...il.com,
        josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, linux-kernel@...r.kernel.org,
        kernel-team@....com, peterz@...radead.org
Subject: Re: [PATCH] rcu: Report a quiescent state when it's exactly in the
 state



On 2018-05-15 06:04, Paul E. McKenney wrote:
> On Mon, May 14, 2018 at 11:59:41AM +0900, Byungchul Park wrote:
>> 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@....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. :)
> 
> Given what it currently does, the name should be rcu_tasks_qs() to go
> along with rcu_bh_qs(), rcu_preempt_qs(), and rcu_sched_qs().  Much as
> I would like cond_resched() to be an RCU-tasks quiescent state, it is
> nothingness for PREEMPT=y kernels, and Peter has indicated a strong
> interest in having it remain so.  But I did update a few comments.
> 
> I left rcu_note_voluntary_context_switch() alone because it should be
> disappearing entirely Real Soon Now.
> 
> Please see patch below.
> 
> 							Thanx, Paul
> 
> PS.  Oddly enough, the recent patch removing the "if" from
>       cond_resched_tasks_rcu_qs() is (technically speaking) pointless.
>       If the kernel contains RCU-tasks, it must be preemptible, which
>       means that cond_resched() unconditionally returns false, which
>       in turn means that rcu_note_voluntary_context_switch_lite() was
>       unconditionally invoked.
> 
>       Simiarly, in non-preemptible kernels, where cond_resched()
>       might well return true, rcu_note_voluntary_context_switch_lite()
>       is a no-op.

Interesting. Right. Thanks for your explanation. :)

-- 
Thanks,
Byungchul