linux-kernel - Re: [PATCH 0/2] arm64: Allow the rescheduling IPI to bypass irq

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Date:   Tue, 10 Nov 2020 14:03:48 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Marc Zyngier <maz@...nel.org>,
        LAK <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH 0/2] arm64: Allow the rescheduling IPI to bypass
 irq_enter/exit

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.