linux-kernel - Re: smp_call_function

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Message-ID: <CA+55aFwWT--5mgKqryfFAbgaoEacsZn8dZ0POWH3xpdNgRMuRw@mail.gmail.com>
Date:	Thu, 19 Feb 2015 08:26:23 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Rafael David Tinoco <inaddy@...ntu.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Gema Gomez <gema.gomez-solano@...onical.com>,
	chris.j.arges@...onical.com,
	"the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: smp_call_function_single lockups

On Thu, Feb 19, 2015 at 7:42 AM, Rafael David Tinoco <inaddy@...ntu.com> wrote:
>
> Same environment as before: Nested KVM (2 vcpus) on top of Proliant
> DL380G8 with acpi_idle and no x2apic optout.

Btw, which apic model does that end up using? Does "no x2apic optout"
mean you're using the x2apic?

What does "dmesg | grep apic" report? Something like

    Switched APIC routing to cluster x2apic.

or what?

Side note to the apic guys: I think the "single CPU" case ends up
being one of the most important ones, but the stupid APIC model
doesn't allow that, so sending an IPI to a single CPU ends up being
"send a mask with a single bit set", and then we have that horrible
"for_each_cpu(cpu, mask)" crap.

Would it make sense to perhaps add a "send_IPI_single()" function
call, and then for the APIC models that always are based on masks, use
a wrapper that just does that "cpumask_of(cpu)" thing..

Hmm?

                       Linus
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