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Re: [Xen-devel] [PATCH v8 00/10] qspinlock: a 4-byte queue spinlock with PV support

On 04/04/2014 12:55 PM, Konrad Rzeszutek Wilk wrote:
On Thu, Apr 03, 2014 at 10:57:18PM -0400, Waiman Long wrote:
On 04/03/2014 01:23 PM, Konrad Rzeszutek Wilk wrote:
On Wed, Apr 02, 2014 at 10:10:17PM -0400, Waiman Long wrote:
On 04/02/2014 04:35 PM, Waiman Long wrote:
On 04/02/2014 10:32 AM, Konrad Rzeszutek Wilk wrote:
On Wed, Apr 02, 2014 at 09:27:29AM -0400, Waiman Long wrote:
N.B. Sorry for the duplicate. This patch series were resent as the
      original one was rejected by the vger.kernel.org list server
      due to long header. There is no change in content.

   - Remove one unneeded atomic operation from the slowpath, thus
     improving performance.
   - Simplify some of the codes and add more comments.
   - Test for X86_FEATURE_HYPERVISOR CPU feature bit to enable/disable
     unfair lock.
   - Reduce unfair lock slowpath lock stealing frequency depending
     on its distance from the queue head.
   - Add performance data for IvyBridge-EX CPU.
FYI, your v7 patch with 32 VCPUs (on a 32 cpu socket machine) on an
HVM guest under Xen after a while stops working. The workload
is doing 'make -j32' on the Linux kernel.

Completely unresponsive. Thoughts?

Thank for reporting that. I haven't done that much testing on Xen.
My focus was in KVM. I will perform more test on Xen to see if I
can reproduce the problem.

BTW, does the halting and sending IPI mechanism work in HVM? I saw
that in RHEL7, PV spinlock was explicitly disabled when in HVM mode.
However, this piece of code isn't in upstream code. So I wonder if
there is problem with that.
The PV ticketlock fixed it for HVM. It was disabled before because
the PV guests were using bytelocks while the HVM were using ticketlocks
and you couldnt' swap in PV bytelocks for ticketlocks during startup.
The RHEL7 code has used PV ticketlock already. RHEL7 uses a single
kernel for all configurations. So PV ticketlock as well as Xen and
KVM support was compiled in. I think booting the kernel on bare
metal will cause the Xen code to work in HVM mode thus activating
the PV spinlock code which has a negative impact on performance.

That may be why it was disabled so that the bare metal performance
will not be impacted.
I am not following you.

What I am saying is that when XEN and PV spinlock is compiled into the current upstream kernel, the PV spinlock jump label is turned on when booted on bare metal. In other words, the PV spinlock code is active even when they are not needed and actually slow thing down in that situation. This is a problem and we need to find way to make sure that the PV spinlock code won't be activated on bare metal.


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