linux-kernel - Re: [PATCH tip/core/rcu 18/18] rcu: Better hotplug handling for synchronize_sched

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Message-ID: <20151007142627.GE3604@twins.programming.kicks-ass.net>
Date:	Wed, 7 Oct 2015 16:26:27 +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,
	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, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH tip/core/rcu 18/18] rcu: Better hotplug handling for
 synchronize_sched_expedited()

On Tue, Oct 06, 2015 at 09:29:37AM -0700, Paul E. McKenney wrote:
>  void rcu_sched_qs(void)
>  {
> +	unsigned long flags;
> +
>  	if (__this_cpu_read(rcu_sched_data.cpu_no_qs.s)) {
>  		trace_rcu_grace_period(TPS("rcu_sched"),
>  				       __this_cpu_read(rcu_sched_data.gpnum),
>  				       TPS("cpuqs"));
>  		__this_cpu_write(rcu_sched_data.cpu_no_qs.b.norm, false);
> +		if (!__this_cpu_read(rcu_sched_data.cpu_no_qs.b.exp))
> +			return;
> +		local_irq_save(flags);
>  		if (__this_cpu_read(rcu_sched_data.cpu_no_qs.b.exp)) {
>  			__this_cpu_write(rcu_sched_data.cpu_no_qs.b.exp, false);
>  			rcu_report_exp_rdp(&rcu_sched_state,
>  					   this_cpu_ptr(&rcu_sched_data),
>  					   true);
>  		}
> +		local_irq_restore(flags);
>  	}
>  }

*sigh*.. still rare I suppose, but should we look at doing something
like this?

---
 kernel/sched/core.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index fe819298c220..3d830c3491c4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3050,7 +3050,6 @@ static void __sched __schedule(void)
 
 	cpu = smp_processor_id();
 	rq = cpu_rq(cpu);
-	rcu_note_context_switch();
 	prev = rq->curr;
 
 	schedule_debug(prev);
@@ -3058,13 +3057,16 @@ static void __sched __schedule(void)
 	if (sched_feat(HRTICK))
 		hrtick_clear(rq);
 
+	local_irq_disable();
+	rcu_note_context_switch();
+
 	/*
 	 * Make sure that signal_pending_state()->signal_pending() below
 	 * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
 	 * done by the caller to avoid the race with signal_wake_up().
 	 */
 	smp_mb__before_spinlock();
-	raw_spin_lock_irq(&rq->lock);
+	raw_spin_lock(&rq->lock);
 	lockdep_pin_lock(&rq->lock);
 
 	rq->clock_skip_update <<= 1; /* promote REQ to ACT */
--
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