From owner-freebsd-current@freebsd.org  Fri Aug 21 16:23:15 2015
Return-Path: <owner-freebsd-current@freebsd.org>
Delivered-To: freebsd-current@mailman.ysv.freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org
 [IPv6:2001:1900:2254:206a::19:1])
 by mailman.ysv.freebsd.org (Postfix) with ESMTP id DD5E39BF9AC
 for <freebsd-current@mailman.ysv.freebsd.org>;
 Fri, 21 Aug 2015 16:23:15 +0000 (UTC) (envelope-from ian@freebsd.org)
Received: from pmta2.delivery6.ore.mailhop.org
 (pmta2.delivery6.ore.mailhop.org [54.200.129.228])
 (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits))
 (Client did not present a certificate)
 by mx1.freebsd.org (Postfix) with ESMTPS id BB43D62F
 for <freebsd-current@freebsd.org>; Fri, 21 Aug 2015 16:23:15 +0000 (UTC)
 (envelope-from ian@freebsd.org)
Received: from ilsoft.org (unknown [73.34.117.227])
 by outbound2.ore.mailhop.org (Halon Mail Gateway) with ESMTPSA;
 Fri, 21 Aug 2015 16:24:44 +0000 (UTC)
Received: from rev (rev [172.22.42.240])
 by ilsoft.org (8.14.9/8.14.9) with ESMTP id t7LGNDhA009208;
 Fri, 21 Aug 2015 10:23:13 -0600 (MDT) (envelope-from ian@freebsd.org)
Message-ID: <1440174193.242.279.camel@freebsd.org>
Subject: Re: freebsd-head: suddenly NMI panics lead to ddb being unable to
 stop CPUs?
From: Ian Lepore <ian@freebsd.org>
To: Julian Elischer <julian@freebsd.org>
Cc: Adrian Chadd <adrian@freebsd.org>, freebsd-current
 <freebsd-current@freebsd.org>
Date: Fri, 21 Aug 2015 10:23:13 -0600
In-Reply-To: <55D74402.70104@freebsd.org>
References: <CAJ-VmomvqULP--v47qKJisQkf8VQNvxEhXK=HXEtv9MuLz4D1g@mail.gmail.com>
 <CAFMmRNw6tWMQ-pfXzSpEM7kRgKafB9KnK-oUhWw2_E-P91drLw@mail.gmail.com>
 <55D74193.4020008@FreeBSD.org>
 <CAJ-Vmon6xXBSMPWgNhg-RZKLuuMDP1hvXG+DdZ3fZdvFnan06g@mail.gmail.com>
 <55D74402.70104@freebsd.org>
Content-Type: text/plain; charset="us-ascii"
X-Mailer: Evolution 3.12.10 FreeBSD GNOME Team Port 
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
X-BeenThere: freebsd-current@freebsd.org
X-Mailman-Version: 2.1.20
Precedence: list
List-Id: Discussions about the use of FreeBSD-current
 <freebsd-current.freebsd.org>
List-Unsubscribe: <https://lists.freebsd.org/mailman/options/freebsd-current>, 
 <mailto:freebsd-current-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-current/>
List-Post: <mailto:freebsd-current@freebsd.org>
List-Help: <mailto:freebsd-current-request@freebsd.org?subject=help>
List-Subscribe: <https://lists.freebsd.org/mailman/listinfo/freebsd-current>, 
 <mailto:freebsd-current-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Fri, 21 Aug 2015 16:23:16 -0000

On Fri, 2015-08-21 at 23:30 +0800, Julian Elischer wrote:
> On 8/21/15 11:25 PM, Adrian Chadd wrote:
> > Ah, cool. I'll give it a whirl.
> >
> > I'm a little worried about having all of the other cores spinning in
> > this case (mostly thermal; the machines get VERY LOUD when the CPUs
> > are spinning..)
> >
> make each spin with the pause instruction.. and for N seconds (N being 
> the CPU ID) or something

cpu_spinwait() is defined as ia32_pause() on i386 and amd64.

The problem is that ONLY x86 arches benefit from cpu_spinwait(), it's
just a nop on other arches, which turn into little radiant heaters for
long spins (like while waiting for APs to be released).

On ARM we have a really nice wait-for-event/signal-event pair of
instructions, where WFE would be a natural for cpu_spinwait(), but would
require a matching cpu_spinwake() type call to be sprinkled around to do
the matching SEV instruction.

-- Ian