From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: [PATCHv1] x86: only check for one watchdog NMI
Date: Tue, 23 Jun 2015 10:34:44 +0100
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References: <1434990111-14360-1-git-send-email-david.vrabel@citrix.com>
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To: Jan Beulich <JBeulich@suse.com>, David Vrabel <david.vrabel@citrix.com>
Cc: xen-devel@lists.xenproject.org, Keir Fraser <keir@xen.org>
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On 23/06/15 10:09, Jan Beulich wrote:
>>>> On 22.06.15 at 18:21, <david.vrabel@citrix.com> wrote:
>> Since the NMI handler can now recognize watchdog NMIs, make
>> check_nmi_watchdog() only check for at least one watchdog NMI.  This
>> prevents false negatives caused by other processors (which may be
>> being power managed by the BIOS) running at reduced clock frequencies.
> Doesn't this go a little too far, in that it allows through the case where
> an NMI works exactly once (a behavior not unheard of)? Lowering the
> count to e.g. 3 would seem acceptable, but not any further. Raising
> the wait time might then need to be the way to go if the (approximate)
> 1:3 ratio still isn't enough to cope with BIOS power managed CPUs.
>
> Btw., can an OS know of the power state CPUs come up in? I.e. can
> the wait time be adjusted dynamically? Or is this (perhaps intentionally)
> completely transparent to the OS?

With the mwait driver, probably, as it is specifically designed to
completely bypass any firmware settings which might be in place.

Anything else is dependent on how much information can be gleaned from
the ACPI tables, but most firmware deliberately has a "BIOS controlled"
mode which is designed to restrict what the OS is capable of doing.

~Andrew