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

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Date:   Fri, 11 May 2018 21:57:54 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     jiangshanlai@...il.com, paulmck@...ux.vnet.ibm.com,
        josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com
Cc:     linux-kernel@...r.kernel.org, kernel-team@....com
Subject: Re: [PATCH] rcu: Report a quiescent state when it's exactly in the
 state

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(); \
>   } while (0)
>   
>   /*
> diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
> index a64eee0..68d2332 100644
> --- a/kernel/rcu/tiny.c
> +++ b/kernel/rcu/tiny.c
> @@ -120,12 +120,12 @@ void rcu_bh_qs(void)
>    */
>   void rcu_check_callbacks(int user)
>   {
> -	if (user)
> +	if (user) {
>   		rcu_sched_qs();
> -	else if (!in_softirq())
>   		rcu_bh_qs();
> -	if (user)
>   		rcu_note_voluntary_context_switch(current);
> +	} else if (!in_softirq())
> +		rcu_bh_qs();
>   }
>   
>   /*
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 36075dd..1abe29a 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2595,6 +2595,7 @@ void rcu_check_callbacks(int user)
>   
>   		rcu_sched_qs();
>   		rcu_bh_qs();
> +		rcu_note_voluntary_context_switch(current);
>   
>   	} else if (!in_softirq()) {
>   
> @@ -2610,8 +2611,7 @@ void rcu_check_callbacks(int user)
>   	rcu_preempt_check_callbacks();
>   	if (rcu_pending())
>   		invoke_rcu_core();
> -	if (user)
> -		rcu_note_voluntary_context_switch(current);
> +
>   	trace_rcu_utilization(TPS("End scheduler-tick"));
>   }
>   
> 

-- 
Thanks,
Byungchul