linux-kernel - Re: [PATCH] arm: Set hardirq tracing to on when idling

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Message-ID: <53852D96.60006@acm.org>
Date:	Tue, 27 May 2014 19:28:06 -0500
From:	Corey Minyard <minyard@....org>
To:	Arnd Bergmann <arnd@...db.de>, linux-arm-kernel@...ts.infradead.org
CC:	Stephen Boyd <sboyd@...eaurora.org>,
	Corey Minyard <cminyard@...sta.com>,
	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	Stanislav Meduna <stano@...una.org>
Subject: Re: [PATCH] arm: Set hardirq tracing to on when idling

On 05/27/2014 02:27 PM, Arnd Bergmann wrote:
> On Tuesday 27 May 2014 11:53:59 Stephen Boyd wrote:
>> On 05/27/14 11:49, Arnd Bergmann wrote:
>>> You also commented in that thread about stop_critical_timings()/
>>> start_critical_timings(). Corey, can you look at that, too? I
>>> think it's designed to avoid the issue you are seeing but
>>> for some reason doesn't.
>> I sent a patch last week to "solve" this problem. I'm not sure if it's
>> right but it works for me.
>>
>> https://lkml.org/lkml/2014/5/19/607
> I think that one was also wrong, as the intention of the existing
> stop_critical_timings() function is already to do the same that
> Corey's patch does, i.e. stop the trace before we go to idle as
> if we were turning IRQs on.
>
> Corey, does it work for you if you replace the new trace_hardirqs_on()
> you added with time_hardirqs_on() or stop_critical_timing()?

Well, more information on this.  It turns out that the generic idle loop
calls stop_critical_timing() and start_critical timing(), so the
arch_cpu_idle() shouldn't have to.

However, the idle loop calls rcu_idle_enter() after it calls
stop_critical_timing(), and that is resetting the critical timing, it
appears.  It's disabling/enabling interrupts in rcu_idle_enter().  If I
switch the order of the rcu_idle and critical timing calls, the issue
goes away.

Stephen's patch does not seem to be necessary for my issue. I tried with
the patch applied, too.  It doesn't seem to hurt, at least.  It did not
fix the problem by itself, though.

-corey
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