Message-ID: <547578C8.7020703@suse.com>
Date: Wed, 26 Nov 2014 07:52:56 +0100
From: Juergen Gross <jgross@suse.com>
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To: Linus Torvalds <torvalds@linux-foundation.org>
CC: the arch/x86 maintainers <x86@kernel.org>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Dave Jones <davej@redhat.com>
Subject: Re: frequent lockups in 3.18rc4
References: <20141114213124.GB3344@redhat.com>	<CA+55aFx16ks_azU0eAcx+qZ5-o8Xo0zzP6aNjrskai8rqp+eMg@mail.gmail.com>	<20141115213405.GA31971@redhat.com>	<20141116014006.GA5016@redhat.com>	<CA+55aFwLH6zGf7O=o2+H0Z8=8nW4zwUbw3Q4bJGttHACv9g+FQ@mail.gmail.com>	<20141126002501.GA11752@redhat.com>	<CA+55aFy9c2FY2gDw14VU537QJoLHAbf+q_75VUwU33_HLz+F0w@mail.gmail.com>	<5475596A.9010301@suse.com>	<CA+55aFx1SiFBzmA=k9jHxi3cZE3Ei_+2NHepujgf86KEvkz8eQ@mail.gmail.com>	<54756424.6020409@suse.com>	<CA+55aFzVyTi_pR4y3=Z3y1bWkXbvgoJaykoyn0pUE7eTYgr+ZQ@mail.gmail.com> <CA+55aFzOSfdvgrkfiN=zZg4mSoot-tJQiVRh5ENW2-rzmLMy1w@mail.gmail.com>
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On 11/26/2014 07:21 AM, Linus Torvalds wrote:
> On Tue, Nov 25, 2014 at 9:52 PM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>>
>> And leave it running for a while, and see if the trace is always the
>> same, or if there are variations on it...
>
> Amusing.
>
> Lookie here:
>
>     http://lists.xenproject.org/archives/html/xen-changelog/2005-08/msg00310.html
>
> That's from 2005.

:-)

>
> Anyway, I don't see why the cr3 issue matters, *unless* there is some
> situation where the scheduler can run with interrupts enabled. And why
> this is Xen-related, I have no idea.
>
> The Xen patches seem to have lost that
>
>   /* On Xen the line below does not always work. Needs investigating! */
>
> line when backporting the 2.6.29 patches to Xen. And clearly nobody
> investigated.
>
> So please do get me back-traces, and we'll investigate. Better late
> than never. But it does sound Xen-specific - although it's possible
> that Xen just triggers some timing (and has apparently been able to
> trigger it since 2005) that DaveJ now triggers on his one machine.

Yeah, this sounds plausible.

I'm working on the back traces right now, hope to have them soon.


Juergen

>
> So DaveJ, even though this does appear Xen-centric (Xentric?) and
> you're running on bare hardware, maybe you could do the same thing in
> that x86-64 vmalloc_fault(). The timing with Jürgen is kind of
> intriguing - if 3.18-rc made it happen much more often for him, maybe
> it really is very timing-sensitive, and you actually are seeing a
> non-Xen version of the same thing...
>
>                             Linus
>

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