linux-kernel - Re: [PATCH RFC tip/core/rcu 1/9] rcu: Add call_rcu

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Message-ID: <20140729075055.GY19379@twins.programming.kicks-ass.net>
Date:	Tue, 29 Jul 2014 09:50:55 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.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, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH RFC tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

On Mon, Jul 28, 2014 at 03:56:12PM -0700, Paul E. McKenney wrote:
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index bc1638b33449..a0d2f3a03566 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2762,6 +2762,7 @@ need_resched:
>  		} else {
>  			deactivate_task(rq, prev, DEQUEUE_SLEEP);
>  			prev->on_rq = 0;
> +			rcu_note_voluntary_context_switch(prev);
>  
>  			/*
>  			 * If a worker went to sleep, notify and ask workqueue
> @@ -2828,6 +2829,7 @@ asmlinkage __visible void __sched schedule(void)
>  	struct task_struct *tsk = current;
>  
>  	sched_submit_work(tsk);
> +	rcu_note_voluntary_context_switch(tsk);
>  	__schedule();
>  }

Yeah, not entirely happy with that, you add two calls into one of the
hotest paths of the kernel.

And I'm still not entirely sure why, your 0/x babbled something about
trampolines, but I'm not sure I understand how those lead to this.

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