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 15:21:39 -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>
Subject: Re: [PATCH v2] tracing: Don't account for cpu idle time with irqsoff
 tracers

On 05/27/14 14:48, Steven Rostedt wrote:
> On Tue, 27 May 2014 22:11:25 +0200
> Arnd Bergmann <arnd@...db.de> wrote:
>
>> On Tuesday 27 May 2014 13:08:04 Stephen Boyd wrote:
>>> @@ -380,7 +382,7 @@ start_critical_timing(unsigned long ip, unsigned long parent_ip)
>>>  
>>>         cpu = raw_smp_processor_id();
>>>  
>>> -       if (per_cpu(tracing_cpu, cpu))
>>> +       if (per_cpu(timings_stopped, cpu) || per_cpu(tracing_cpu, cpu))
>>>                 return;
>>>  
>>>         data = per_cpu_ptr(tr->trace_buffer.data, cpu);
>>>
>> Where exactly do you see other code calling here while
>> per_cpu(timings_stopped) is set? Would it be possible to just
>> change that call site?
> Arnd brings up a good point. 

Hrm.. still not getting Arnd's mails.

> 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.

What about clockevents_notify? __raw_spin_lock_irqsave() should probably
use raw_local_irqsave().

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.

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

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