linux-kernel - Re: [PATCH v2] tracing: Don't account for cpu idle time with irqsoff tracers

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Date:	Tue, 27 May 2014 17:23:01 -0700
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Arnd Bergmann <arnd@...db.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Corey Minyard <cminyard@...sta.com>,
	Stanislav Meduna <stano@...una.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2] tracing: Don't account for cpu idle time with irqsoff
 tracers

On 05/27/14 17:11, Stephen Boyd wrote:
> On 05/27/14 16:30, Steven Rostedt wrote:
>> On Tue, 27 May 2014 15:21:39 -0700
>> Stephen Boyd <sboyd@...eaurora.org> wrote:
>>
>>
>>>> Arnd brings up a good point. 
>>> Hrm.. still not getting Arnd's mails.
>> Strange. What mail service do you have. Could they be blocking him?
>>
>>>> If we disable irqs off tracing completely,
>>>> we may be missing places in the idle path that disable interrupts for
>>>> long periods of time. We may want to move the stop down further.
>>>>
>>>> The way it works (IIRC), and why tracing can start again is that it can
>>>> nest. Perhaps we need to stop it further down if we can't move it
>>>> completely.
>>>>
>>> I'm not sure how much deeper it can go and I'm afraid it will become a
>>> game of whack-a-mole. I already see two places that disable and reenable
>>> irqs after stop_critical_timings() is called (first in rcu_idle_enter()
>>> and second in clockevents_notify()). Should rcu_idle_enter() move to
>>> raw_local_irq_save()? It looks like that path calls rcu_sched_qs() and
>>> on tiny RCU that again needs the raw_ treatement. We can probably call
>>> stop_critical_timings() after rcu_idle_enter() to fix this.
>> I don't think we need to whack-a-mole. The start stop should be around
>> where it goes to sleep.
>>
>>> What about clockevents_notify? __raw_spin_lock_irqsave() should probably
>>> use raw_local_irqsave().
>> No that solution is even worse. We need lockdep working here.
>>
>>> If we go the raw route, do we even need stop/start_critical_timings()?
>>> Can't we just use raw accessors in the idle paths
>>> (tick_nohz_idle_{enter,exit}(), cpuidle_enter(), etc.) and get rid of
>>> the stop/start stuff completely? I admit this patch is pretty much a big
>>> sledge hammer that tries to make things simple, but if there is some
>>> benefit to the raw accessors I'm willing to send patches to fix all the
>>> call sites.
>>>
>> How about the following. I don't see any reason stop_critical_timings()
>> can't be called from within rcu_idle code, as it doesn't use any rcu.
>>
>> Paul, Peter, see anything wrong with this?
>>
> cpuidle_enter_state() calls ktime_get() which on lockdep enabled builds
> calls seqcount_lockdep_reader_access() which calls local_irq_save() that
> then turns on the tracer again. Perhaps the problem is that irqsoff
> tracer is triggered even when we aren't transitioning between irqs on
> and irqs off? What about this patch? I assume there is a reason that
> this is wrong, but I don't know what it is.
>
> ---8<-----
>
> diff --git a/include/linux/irqflags.h b/include/linux/irqflags.h
> index d176d658fe25..ac8e0a4968bd 100644
> --- a/include/linux/irqflags.h
> +++ b/include/linux/irqflags.h
> @@ -93,7 +93,8 @@
>  #define local_irq_save(flags)				\
>  	do {						\
>  		raw_local_irq_save(flags);		\
> -		trace_hardirqs_off();			\
> +		if (!raw_irqs_disabled_flags(flags))	\
> +			trace_hardirqs_off();		\
>  	} while (0)
>  
>  
> @@ -101,7 +102,6 @@
>  	do {						\
>  		if (raw_irqs_disabled_flags(flags)) {	\
>  			raw_local_irq_restore(flags);	\
> -			trace_hardirqs_off();		\
>  		} else {				\
>  			trace_hardirqs_on();		\
>  			raw_local_irq_restore(flags);	\
>

Aha, looks like lockdep wants to know about redundant hardirqs with the
redundant_hardirqs_off field. Can we use the same field in the irqsoff
tracer to monitor the hardirq on/off state more accurately?

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
hosted by The Linux Foundation

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