linux-kernel - Re: [PATCH tip/core/rcu 01/23] rcu: Move RCU grace-period initialization into a kthread

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Date:	Thu, 06 Sep 2012 15:32:22 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu, patches@...aro.org
Subject: Re: [PATCH tip/core/rcu 01/23] rcu: Move RCU grace-period
 initialization into a kthread

On Thu, 2012-08-30 at 11:18 -0700, Paul E. McKenney wrote:
> +static int rcu_gp_kthread(void *arg)
> +{
> +       unsigned long flags;
> +       struct rcu_data *rdp;
> +       struct rcu_node *rnp;
> +       struct rcu_state *rsp = arg;
> +
> +       for (;;) {
> +
> +               /* Handle grace-period start. */
> +               rnp = rcu_get_root(rsp);
> +               for (;;) {
> +                       wait_event_interruptible(rsp->gp_wq, rsp->gp_flags);
> +                       if (rsp->gp_flags)
> +                               break;
> +                       flush_signals(current);
> +               }
> +               raw_spin_lock_irqsave(&rnp->lock, flags); 

You're in a kthread, it should be impossible for IRQs to be disabled
here, no? Similar for most (all) other sites in this function.

Using the unconditional IRQ disable/enable is generally faster.
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