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Date:   Wed, 26 Aug 2020 21:24:19 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org, will@...nel.org,
        npiggin@...il.com, elver@...gle.com, jgross@...e.com,
        paulmck@...nel.org, rostedt@...dmis.org, rjw@...ysocki.net,
        svens@...ux.ibm.com, tglx@...utronix.de
Subject: Re: [PATCH v2 03/11] sched,idle,rcu: Push rcu_idle deeper into the
 idle path

On Wed, Aug 26, 2020 at 09:18:26PM -0400, Joel Fernandes wrote:
> On Fri, Aug 21, 2020 at 10:47:41AM +0200, Peter Zijlstra wrote:
> > Lots of things take locks, due to a wee bug, rcu_lockdep didn't notice
> > that the locking tracepoints were using RCU.
> > 
> > Push rcu_idle_{enter,exit}() as deep as possible into the idle paths,
> > this also resolves a lot of _rcuidle()/RCU_NONIDLE() usage.
> > 
> > Specifically, sched_clock_idle_wakeup_event() will use ktime which
> > will use seqlocks which will tickle lockdep, and
> > stop_critical_timings() uses lock.
> 
> I was wondering if those tracepoints should just use _rcuidle variant of the
> trace call. But that's a terrible idea considering that would add unwanted
> overhead I think.
> 
> Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>

BTW, if tracepoint is converted to use RCU-trace flavor, then these kinds of
issues go away, no? That RCU flavor is always watching.

thanks,

 - Joel

> 
> thanks,
> 
>  - Joel
> 
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> > Tested-by: Marco Elver <elver@...gle.com>
> > ---
> >  drivers/cpuidle/cpuidle.c |   12 ++++++++----
> >  kernel/sched/idle.c       |   22 ++++++++--------------
> >  2 files changed, 16 insertions(+), 18 deletions(-)
> > 
> > --- a/drivers/cpuidle/cpuidle.c
> > +++ b/drivers/cpuidle/cpuidle.c
> > @@ -145,13 +145,14 @@ static void enter_s2idle_proper(struct c
> >  	 * executing it contains RCU usage regarded as invalid in the idle
> >  	 * context, so tell RCU about that.
> >  	 */
> > -	RCU_NONIDLE(tick_freeze());
> > +	tick_freeze();
> >  	/*
> >  	 * The state used here cannot be a "coupled" one, because the "coupled"
> >  	 * cpuidle mechanism enables interrupts and doing that with timekeeping
> >  	 * suspended is generally unsafe.
> >  	 */
> >  	stop_critical_timings();
> > +	rcu_idle_enter();
> >  	drv->states[index].enter_s2idle(dev, drv, index);
> >  	if (WARN_ON_ONCE(!irqs_disabled()))
> >  		local_irq_disable();
> > @@ -160,7 +161,8 @@ static void enter_s2idle_proper(struct c
> >  	 * first CPU executing it calls functions containing RCU read-side
> >  	 * critical sections, so tell RCU about that.
> >  	 */
> > -	RCU_NONIDLE(tick_unfreeze());
> > +	rcu_idle_exit();
> > +	tick_unfreeze();
> >  	start_critical_timings();
> >  
> >  	time_end = ns_to_ktime(local_clock());
> > @@ -229,16 +231,18 @@ int cpuidle_enter_state(struct cpuidle_d
> >  	/* Take note of the planned idle state. */
> >  	sched_idle_set_state(target_state);
> >  
> > -	trace_cpu_idle_rcuidle(index, dev->cpu);
> > +	trace_cpu_idle(index, dev->cpu);
> >  	time_start = ns_to_ktime(local_clock());
> >  
> >  	stop_critical_timings();
> > +	rcu_idle_enter();
> >  	entered_state = target_state->enter(dev, drv, index);
> > +	rcu_idle_exit();
> >  	start_critical_timings();
> >  
> >  	sched_clock_idle_wakeup_event();
> >  	time_end = ns_to_ktime(local_clock());
> > -	trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, dev->cpu);
> > +	trace_cpu_idle(PWR_EVENT_EXIT, dev->cpu);
> >  
> >  	/* The cpu is no longer idle or about to enter idle. */
> >  	sched_idle_set_state(NULL);
> > --- a/kernel/sched/idle.c
> > +++ b/kernel/sched/idle.c
> > @@ -54,17 +54,18 @@ __setup("hlt", cpu_idle_nopoll_setup);
> >  
> >  static noinline int __cpuidle cpu_idle_poll(void)
> >  {
> > +	trace_cpu_idle(0, smp_processor_id());
> > +	stop_critical_timings();
> >  	rcu_idle_enter();
> > -	trace_cpu_idle_rcuidle(0, smp_processor_id());
> >  	local_irq_enable();
> > -	stop_critical_timings();
> >  
> >  	while (!tif_need_resched() &&
> > -		(cpu_idle_force_poll || tick_check_broadcast_expired()))
> > +	       (cpu_idle_force_poll || tick_check_broadcast_expired()))
> >  		cpu_relax();
> > -	start_critical_timings();
> > -	trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, smp_processor_id());
> > +
> >  	rcu_idle_exit();
> > +	start_critical_timings();
> > +	trace_cpu_idle(PWR_EVENT_EXIT, smp_processor_id());
> >  
> >  	return 1;
> >  }
> > @@ -91,7 +92,9 @@ void __cpuidle default_idle_call(void)
> >  		local_irq_enable();
> >  	} else {
> >  		stop_critical_timings();
> > +		rcu_idle_enter();
> >  		arch_cpu_idle();
> > +		rcu_idle_exit();
> >  		start_critical_timings();
> >  	}
> >  }
> > @@ -158,7 +161,6 @@ static void cpuidle_idle_call(void)
> >  
> >  	if (cpuidle_not_available(drv, dev)) {
> >  		tick_nohz_idle_stop_tick();
> > -		rcu_idle_enter();
> >  
> >  		default_idle_call();
> >  		goto exit_idle;
> > @@ -178,21 +180,17 @@ static void cpuidle_idle_call(void)
> >  		u64 max_latency_ns;
> >  
> >  		if (idle_should_enter_s2idle()) {
> > -			rcu_idle_enter();
> >  
> >  			entered_state = call_cpuidle_s2idle(drv, dev);
> >  			if (entered_state > 0)
> >  				goto exit_idle;
> >  
> > -			rcu_idle_exit();
> > -
> >  			max_latency_ns = U64_MAX;
> >  		} else {
> >  			max_latency_ns = dev->forced_idle_latency_limit_ns;
> >  		}
> >  
> >  		tick_nohz_idle_stop_tick();
> > -		rcu_idle_enter();
> >  
> >  		next_state = cpuidle_find_deepest_state(drv, dev, max_latency_ns);
> >  		call_cpuidle(drv, dev, next_state);
> > @@ -209,8 +207,6 @@ static void cpuidle_idle_call(void)
> >  		else
> >  			tick_nohz_idle_retain_tick();
> >  
> > -		rcu_idle_enter();
> > -
> >  		entered_state = call_cpuidle(drv, dev, next_state);
> >  		/*
> >  		 * Give the governor an opportunity to reflect on the outcome
> > @@ -226,8 +222,6 @@ static void cpuidle_idle_call(void)
> >  	 */
> >  	if (WARN_ON_ONCE(irqs_disabled()))
> >  		local_irq_enable();
> > -
> > -	rcu_idle_exit();
> >  }
> >  
> >  /*
> > 
> > 

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