linux-kernel - Re: frequent lockups in 3.18rc4

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Message-ID: <20141217170139.GB8142@laptop.dumpdata.com>
Date:	Wed, 17 Dec 2014 12:01:39 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mgorman@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Tejun Heo <tj@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Don Zickus <dzickus@...hat.com>, Dave Jones <davej@...hat.com>,
	the arch/x86 maintainers <x86@...nel.org>
Subject: Re: frequent lockups in 3.18rc4

On Tue, Dec 16, 2014 at 04:41:16PM -0800, Andy Lutomirski wrote:
> On Tue, Dec 16, 2014 at 4:00 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> > On Tue, Dec 16, 2014 at 3:02 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> >>
> >> OK, should we just stick it in the x86 tree and see if anything
> >> explodes? ;-)
> >
> > Gaah, I got confused about the patches.
> >
> > And something did explode, it showed some Xen nasties. Xen has that
> > odd "we don't share PMD entries between MM's" thing going on, which
> > means that the vmalloc fault thing does actually have to occasionally
> > walk two levels rather than just copy the top level. I'm still not
> > sure why Xen doesn't share PMD's, since threads that shame the MM
> > clearly can share PMD's within Xen, but I gave up on it.
> 
> Sounds like it's time to ask Konrad, the source of all Xen understanding :)

Awesome :-)
> 
> Linus, do you have a pointer to whatever version of the patch you tried?

The patch was this:

a) http://article.gmane.org/gmane.linux.kernel/1835331

Then Jurgen had a patch:
https://lkml.kernel.org/g/CA+55aFxSRujj=cM1NkXYvxmo=Y1hb1e3tgLhdh1JDphzV6WKRw@mail.gmail.com
which was one fix for one bug that ended up being fixed in QEMU - so
it can be ignored.

But my understanding of that thread was that it said patch 'a)' did not
fix Dave's issues - and the conversation went off on NMI watchdog?

I will look up the giant thread to make sense.


> 
> --Andy
> 
> >
> > That said, making x86-64 use "read_cr3()" instead of
> > "current->active_mm" would at least make things a bit safer wrt NMI's
> > during the task switch, of course.  So *some* 32/64-bit consolidation
> > should be done, but my patch went a bit too far for Xen.
> >
> >                       Linus
> 
> 
> 
> -- 
> Andy Lutomirski
> AMA Capital Management, LLC
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