linux-kernel - Re: [PATCH tip/core/rcu 10/10] rcu: Account for rcu_all_qs() in cond

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Message-ID: <20171202085626.GS3326@worktop>
Date:   Sat, 2 Dec 2017 09:56:26 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com
Subject: Re: [PATCH tip/core/rcu 10/10] rcu: Account for rcu_all_qs() in
 cond_resched()

On Fri, Dec 01, 2017 at 11:21:44AM -0800, Paul E. McKenney wrote:
> If cond_resched() returns false, then it has already invoked
> rcu_all_qs().  This is also invoked (now redundantly) by
> rcu_note_voluntary_context_switch().  This commit therefore changes
> cond_resched_rcu_qs() to invoke rcu_note_voluntary_context_switch_lite()
> instead of rcu_note_voluntary_context_switch() to avoid the redundant
> invocation of rcu_all_qs().
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
>  include/linux/rcupdate.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index a6ddc42f87a5..7bd8b5a6db10 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -197,7 +197,7 @@ static inline void exit_tasks_rcu_finish(void) { }
>  #define cond_resched_rcu_qs() \
>  do { \
>  	if (!cond_resched()) \
> -		rcu_note_voluntary_context_switch(current); \
> +		rcu_note_voluntary_context_switch_lite(current); \
>  } while (0)
>  

Maybe I'm confused, but why are we keeping cond_resched_rcu_qs() around
at all?