linux-kernel - Re: [GIT pull] locking/urgent for v5.10-rc6

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Message-ID: <CAHk-=whPM9Ng6OsNGy==6F6WqEqLFo3kVwS1Hs063woxah5Z5g@mail.gmail.com>
Date:   Mon, 30 Nov 2020 09:55:44 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [GIT pull] locking/urgent for v5.10-rc6

On Sun, Nov 29, 2020 at 11:57 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> The idea was to flip all of arch_cpu_idle() to not enable interrupts.

I don't think that's realistic.

> This is suboptimal for things like x86 where arch_cpu_idle() is
> basically STI;HLT, but x86 isn't likely to actually use this code path
> anyway, given all the various cpuidle drivers it has.

Well, but the thing is, "enable interrupts" is pretty much fundamental
to any idle routine.

Idling with interrupts disabled is not a sensible operation. The fact
that on x86 the sequence is "sti;hlt" is not just some x86 oddity,
it's basically fundamental to the whole notion of idle.

Yes, yes, I can very well imagine some hardware doing a "idle until
you sense an interrupt, but don't actually take it". It's not
_impossible_. But it's certainly not normal.

So I think it's completely misguided to think that the default idle
routine should assume that arch_cpu_idle() wouldn't enable interrupts.

             Linus