linux-kernel - Re: Loongson (and other $ARCHs?) idle VS timer enqueue

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Message-ID: <20230422080814.GA1214746@hirez.programming.kicks-ass.net>
Date:   Sat, 22 Apr 2023 10:08:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Huacai Chen <chenhuacai@...nel.org>,
        WANG Xuerui <kernel@...0n.name>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Loongson (and other $ARCHs?) idle VS timer enqueue

On Fri, Apr 21, 2023 at 06:47:29PM +0200, Frederic Weisbecker wrote:

> > *HOWEVER*
> > 
> > intel_idle_irq() is affected -- except that we only (normally) use that
> > for very shallow idle states and it won't interact with NOHZ (because we
> > only disable the tick for deep idle states).
> 
> Well I don't know, that doesn't look comfortable... :)
> 
> Also why does it need to enable IRQs if ecx=1 ?

Supposedly this is some interrupt latency hack. See commit:

  c227233ad64c ("intel_idle: enable interrupts before C1 on Xeons")