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

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Date:	Fri, 4 Apr 2014 12:55:24 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Waiman Long <waiman.long@...com>,
	Marcos Matsunaga <Marcos.Matsunaga@...cle.com>
Cc:	Marcos Matsunaga <Marcos.Matsunaga@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	Paolo Bonzini <paolo.bonzini@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>,
	Chegu Vinod <chegu_vinod@...com>, boris.ostrovsky@...cle.com
Subject: Re: [PATCH v8 00/10] qspinlock: a 4-byte queue spinlock with PV
 support

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.
> >>>>>
> >>>>>v7->v8:
> >>>>>   - 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
> >Yes.
> >>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.

Huh? -EPARSE

> That may be why it was disabled so that the bare metal performance
> will not be impacted.

I am not following you.
> 
> BTW, could you send me more information about the configuration of
> the machine, like the .config file that you used?

Marcos, could you please send that information to Peter. Thanks!
> 
> -Longman
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