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Re: [Xen-devel] [PATCH RFC v5 7/8] pvqspinlock, x86: Add qspinlock para-virtualization support
- To: Paolo Bonzini <pbonzini@xxxxxxxxxx>, David Vrabel <david.vrabel@xxxxxxxxxx>, Waiman Long <Waiman.Long@xxxxxx>
- From: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
- Date: Thu, 27 Feb 2014 20:52:57 +0530
- Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx, Andi Kleen <andi@xxxxxxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, Michel Lespinasse <walken@xxxxxxxxxx>, Alok Kataria <akataria@xxxxxxxxxx>, linux-arch@xxxxxxxxxxxxxxx, x86@xxxxxxxxxx, Ingo Molnar <mingo@xxxxxxxxxx>, Scott J Norton <scott.norton@xxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>, Alexander Fyodorov <halcy@xxxxxxxxx>, Arnd Bergmann <arnd@xxxxxxxx>, Daniel J Blueman <daniel@xxxxxxxxxxxxx>, Rusty Russell <rusty@xxxxxxxxxxxxxxx>, Oleg Nesterov <oleg@xxxxxxxxxx>, Steven Rostedt <rostedt@xxxxxxxxxxx>, Chris Wright <chrisw@xxxxxxxxxxxx>, George Spelvin <linux@xxxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Aswin Chandramouleeswaran <aswin@xxxxxx>, Chegu Vinod <chegu_vinod@xxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
- Delivery-date: Thu, 27 Feb 2014 15:17:39 +0000
- List-id: Xen developer discussion <xen-devel.lists.xen.org>
On 02/27/2014 08:15 PM, Paolo Bonzini wrote:
[...]
But neither of the VCPUs being kicked here are halted -- they're either
running or runnable (descheduled by the hypervisor).
/me actually looks at Waiman's code...
Right, this is really different from pvticketlocks, where the *unlock*
primitive wakes up a sleeping VCPU. It is more similar to PLE
(pause-loop exiting).
Adding to the discussion, I see there are two possibilities here,
considering that in undercommit cases we should not exceed
HEAD_SPIN_THRESHOLD,
1. the looping vcpu in pv_head_spin_check() should do halt()
considering that we have done enough spinning (more than typical
lock-hold time), and hence we are in potential overcommit.
2. multiplex kick_cpu to do directed yield in qspinlock case.
But this may result in some ping ponging?
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