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Subject: Re: How to know if there is a pending interrupt when they are masked?
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Marc Zyngier <Marc.Zyngier@arm.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>
References: <bf2e6e81-7218-05e0-fd6e-27c9c5418813@linaro.org>
 <alpine.DEB.2.21.1902141620510.1561@nanos.tec.linutronix.de>
From:   Daniel Lezcano <daniel.lezcano@linaro.org>
Message-ID: <663d069e-55c9-c4f0-dc84-f6145b998192@linaro.org>
Date:   Thu, 14 Feb 2019 17:12:36 +0100
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Hi Thomas,


On 14/02/2019 16:35, Thomas Gleixner wrote:
> On Thu, 14 Feb 2019, Daniel Lezcano wrote:
>> today we have the possibility to mask the local interrupt with the
>> command local_irq_disable / local_irq_enable.
>>
>> The power management path disables the local interrupt in order to
>> initiate a power down sequence. If there is a pending interrupt this one
>> will make the power down function to abort, thus exiting right after
>> calling the shutdown function after costly operations. It could be
>> interesting to check if there is a pending interrupt before initiating
>> the power down sequence.
>>
>> Is there a way to know if there is a pending interrupt on the current
>> CPU when the local interrupt are disabled? Something like,
>> local_irq_pending() function ?
> 
> We have nothing like that today, but it would be possible to implement this
> at least on x86 by peeking the local APIC registers.
> 
> OTOH, the question is whether it's worth the trouble because the interrupt
> could come in right after the query and the same issue which you want to
> address persists. It only makes sense if it reduces the time window so
> significantly that it actually matters.

Yes, I agree. It should be worth only if we can prove we enter the deep
idle sequence with pending interrupts much more often than what we expect.

Actually, we have a similar action when the need_resched() is true, we
abort the idle sequence and exit the loop.


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