Re: [4.2, Regression] Queued spinlocks cause major XFS performance regression

[Davidlohr Bueso]

On Mon, 07 Sep 2015, Linus Torvalds wrote:

> On Sun, Sep 6, 2015 at 11:57 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > Just to continue the argument for arguments sake, the function is named
> > 'virt' (not paravirt) and tests the HYPERVISOR CPUID bit. How is that
> > not appropriately named?
>
> Well, I think right now one issue is that you can't avoid it, even
> when you want pure "raw hardware" spinlocks.
>
> I really think it should at the very least be inside CONFIG_PARAVIRT.

Yeah, I think we all agree here.

> Because it *is* about helping the hypervisor, so really is about
> paravirtualization.

Yes, this is how I interpret it as well.

CONFIG_PARAVIRT seems like a suitable place as while it is known to induce
in overhead for baremetal and distros tend to enable it by default - mainly
for mem pvops (ie: in page fault paths), and having this function doesn't
add any complexity or add make things much different than they already are.

While it is true that CONFIG_HYPERVISOR_GUEST is a pre-req for anything pv,
technically I think it's not as good a fit as CONFIG_PARAVIRT because the
former is really about describing the hypervisor, and that's not what we're
doing here.


Thanks,
Davidlohr
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/