Re: frequent lockups in 3.18rc4

[Josh Boyer]

On Mon, Nov 24, 2014 at 1:48 PM, Konrad Rzeszutek Wilk
<konrad.wilk@xxxxxxxxxx> wrote:
> 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@xxxxxxxxxxxxxx> wrote:
>> > On Fri, Nov 21, 2014 at 12:14 PM, Josh Boyer <jwboyer@xxxxxxxxxxxxxxxxx> wrote:
>> >> On Fri, Nov 21, 2014 at 2:52 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> >>> On Fri, Nov 21, 2014 at 11:46 AM, Linus Torvalds
>> >>> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>> >>>> On Fri, Nov 21, 2014 at 11:34 AM, Linus Torvalds
>> >>>> <torvalds@xxxxxxxxxxxxxxxxxxxx> 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!

It's sad because I can't really play the trombone and it sounds horrible.

> 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@xxxxxxxxxxxxxxxxxxxx
> if that is possible.

Indeed, you have been massively helpful.  My comment on it being not
well paid attention to was a reflection on the distro maintainers, not
you.  You've been great once we notice the Xen issue, but that takes a
while on our part and it isn't the best of user experiences :\.

> 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>).

OK.  I'll try and do better at keeping up with things.

josh
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