linux-kernel - Re: frequent lockups in 3.18rc4

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Message-ID: <20141124184856.GA9349@laptop.dumpdata.com>
Date:	Mon, 24 Nov 2014 13:48:56 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Josh Boyer <jwboyer@...oraproject.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Tejun Heo <tj@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	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 Fri, Nov 21, 2014 at 03:23:13PM -0500, Josh Boyer wrote:
> On Fri, Nov 21, 2014 at 3:16 PM, Andy Lutomirski <luto@...capital.net> wrote:
> > On Fri, Nov 21, 2014 at 12:14 PM, Josh Boyer <jwboyer@...oraproject.org> wrote:
> >> On Fri, Nov 21, 2014 at 2:52 PM, Andy Lutomirski <luto@...capital.net> wrote:
> >>> On Fri, Nov 21, 2014 at 11:46 AM, Linus Torvalds
> >>> <torvalds@...ux-foundation.org> wrote:
> >>>> On Fri, Nov 21, 2014 at 11:34 AM, Linus Torvalds
> >>>> <torvalds@...ux-foundation.org> wrote:
> >>>>>
> >>>>> So I kind of agree, but it wouldn't be my primary worry. My primary
> >>>>> worry is actually paravirt doing something insane.
> >>>>
> >>>> Btw, on that tangent, does anybody actually care about paravirt any more?
> >>>>
> >>>
> >>> Amazon, for better or for worse.

And distros: Oracle and Novell.

> >>>
> >>>> I'd love to start moving away from it. It makes a lot of the low-level
> >>>> code completely impossible to follow due to the random indirection
> >>>> through "native" vs "paravirt op table". Not just the page table
> >>>> handling, it's all over.
> >>>>
> >>>> Anybody who seriously does virtualization uses hw virtualization that
> >>>> is much better than it used to be. And the non-serious users aren't
> >>>> that performance-sensitive by definition.

I would point out that the PV paravirt spinlock gives an huge boost
for virtualization guests (this is for both KVM and Xen).
> >>>>
> >>>> I note that the Fedora kernel config seems to include paravirt by
> >>>> default, so you get a lot of the crazy overheads..

Not that much. We ran benchmarks and it was in i-cache overhead - and
the numbers came out to be sub-1% percent.
> >>>
> >>> I think that there is a move toward deprecating Xen PV in favor of
> >>> PVH, but we're not there yet.
> >>
> >> A move where?  The Xen stuff in Fedora is ... not paid attention to
> >> very much.  If there's something we should be looking at turning off
> >> (or on), we're happy to take suggestions.
> >
> > A move in the Xen project.  As I understand it, Xen wants to deprecate
> > PV in favor of PVH, but PVH is still experimental.
> 
> OK.
> 
> > I think that dropping PARAVIRT in Fedora might be a bad idea for
> > several more releases, since that's likely to break the EC2 images.
> 
> Yes, that's essentially the only reason we haven't looked at disabling
> Xen completely for a while now, so <sad trombone>.

Heh. Didn't know you could play on a trombone!

As I had mentioned in the past - if there are Xen related bugs on
Fedora please CC me on them. Or perhaps CC xen-devel@...ts.xenproject.org
if that is possible.

And as Andy has mentioned - we are moving towards using PVH as a way
to not use the PV MMU ops. But that is still off (<sad trombone played
from YouTube>).

> 
> josh
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