linux-kernel - Re: [PATCH v2 03/11] sched,idle,rcu: Push rcu

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Date:   Wed, 26 Aug 2020 21:18:26 -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 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>

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();
>  }
>  
>  /*
> 
>