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References: <20201101131430.257038-1-maz@kernel.org> <jhjsg9syrs5.mognet@arm.com> <20201110130348.GK2594@hirez.programming.kicks-ass.net> <19286daf276f46aa@fake-msgid>
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From:   Valentin Schneider <valentin.schneider@arm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Marc Zyngier <maz@kernel.org>,
        LAK <linux-arm-kernel@lists.infradead.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Android Kernel Team <kernel-team@android.com>
Subject: Re: [PATCH 0/2] arm64: Allow the rescheduling IPI to bypass irq_enter/exit
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Date:   Tue, 10 Nov 2020 15:48:58 +0000
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On 01/01/70 01:00, Valentin Schneider wrote:
> On 10/11/20 13:03, Peter Zijlstra wrote:
>> On Mon, Nov 02, 2020 at 10:30:50AM +0000, Valentin Schneider wrote:
>>
>>> Now, I'd like to pen exactly why we think it's okay to forgo irq_{enter,
>>> exit}() for that one IRQ and not any other.
>>
>> Thomas already said a few words on this, but basically scheduler_ipi()
>> is a NOP (*almost*), the IPI has no body. All it does is tickle the
>> return-from-interrupt path. So any setup and tear-down done for the
>> non-existing body is a waste of time.

Gotcha.

The pedant in me thinks this makes it more of a handler property than an
IRQ one, but I don't see a nice way to e.g. have this as a flag passed to
__request_percpu_irq() and not have it usable by random modules.