From: Paolo Bonzini <pbonzini@redhat.com>
To: Waiman Long <waiman.long@hp.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>,
Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>,
Peter Zijlstra <peterz@infradead.org>,
virtualization@lists.linux-foundation.org,
Andi Kleen <andi@firstfloor.org>,
"H. Peter Anvin" <hpa@zytor.com>,
Michel Lespinasse <walken@google.com>,
Alok Kataria <akataria@vmware.com>,
linux-arch@vger.kernel.org, x86@kernel.org,
Ingo Molnar <mingo@redhat.com>,
Scott J Norton <scott.norton@hp.com>,
xen-devel@lists.xenproject.org,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Alexander Fyodorov <halcy@yandex.ru>,
Rik van Riel <riel@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
Daniel J Blueman <daniel@numascale.com>,
Oleg Nesterov <oleg@redhat.com>,
Steven Rostedt <rostedt@goodmis.org>,
Chris Wright <chrisw@sous-sol.org>,
George Spelvin <linux@horizon.com>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH RFC v5 4/8] pvqspinlock, x86: Allow unfair spinlock in a real PV environment
Date: Tue, 04 Mar 2014 16:23:16 +0100 [thread overview]
Message-ID: <5315EFE4.6090503@redhat.com> (raw)
In-Reply-To: <5315EE2E.2060002@hp.com>
Il 04/03/2014 16:15, Waiman Long ha scritto:
>>
>> PLE is unnecessary if you have "true" pv spinlocks where the
>> next-in-line schedules itself out with a hypercall (Xen) or hlt
>> instruction (KVM). Set a bit in the qspinlock before going to sleep,
>> and the lock owner will know that it needs to kick the next-in-line.
>>
>> I think there is no need for the unfair lock bits. 1-2% is a pretty
>> large hit.
>
> I don't think that PLE is something that can be controlled by software.
> It is done in hardware.
Yes, but the hypervisor decides *what* to do when the processor detects
a pause-loop.
But my point is that if you have pv spinlocks, the processor in the end
will never or almost never do a pause-loop exit. PLE is mostly for
legacy guests that doesn't have pv spinlocks.
Paolo
> I maybe wrong. Anyway, I plan to add code to
> schedule out the CPUs waiting in the queue except the first 2 in the
> next version of the patch.
>
> The PV code in the v5 patch did seem to improve benchmark performance
> with moderate to heavy spinlock contention. However, I didn't see much
> CPU kicking going on. My theory is that the additional PV code
> complicates the pause loop timing so that the hardware PLE didn't kick
> in, whereas the original pause loop is pretty simple causing PLE to
> happen fairly frequently.
next prev parent reply other threads:[~2014-03-04 15:23 UTC|newest]
Thread overview: 58+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-02-26 15:14 [PATCH v5 0/8] qspinlock: a 4-byte queue spinlock with PV support Waiman Long
2014-02-26 15:14 ` [PATCH v5 1/8] qspinlock: Introducing a 4-byte queue spinlock implementation Waiman Long
2014-02-26 16:22 ` Peter Zijlstra
2014-02-27 20:25 ` Waiman Long
2014-02-26 16:24 ` Peter Zijlstra
2014-02-27 20:25 ` Waiman Long
2014-02-26 15:14 ` [PATCH v5 2/8] qspinlock, x86: Enable x86-64 to use queue spinlock Waiman Long
2014-02-26 15:14 ` [PATCH v5 3/8] qspinlock, x86: Add x86 specific optimization for 2 contending tasks Waiman Long
2014-02-26 16:20 ` Peter Zijlstra
2014-02-27 20:42 ` Waiman Long
2014-02-28 9:29 ` Peter Zijlstra
2014-02-28 16:25 ` Linus Torvalds
2014-02-28 17:37 ` Peter Zijlstra
2014-02-28 16:38 ` Waiman Long
2014-02-28 17:56 ` Peter Zijlstra
2014-03-03 17:43 ` Peter Zijlstra
2014-03-04 15:27 ` Waiman Long
2014-03-04 16:58 ` Peter Zijlstra
2014-03-04 18:09 ` Peter Zijlstra
2014-03-04 17:48 ` Waiman Long
2014-03-04 22:40 ` Peter Zijlstra
2014-03-05 20:59 ` Peter Zijlstra
2014-02-26 15:14 ` [PATCH RFC v5 4/8] pvqspinlock, x86: Allow unfair spinlock in a real PV environment Waiman Long
2014-02-26 17:07 ` Konrad Rzeszutek Wilk
2014-02-28 17:06 ` Waiman Long
2014-03-03 10:55 ` Paolo Bonzini
2014-03-04 15:15 ` Waiman Long
2014-03-04 15:23 ` Paolo Bonzini [this message]
2014-03-04 15:39 ` David Vrabel
2014-03-04 17:50 ` Raghavendra K T
2014-02-27 12:28 ` David Vrabel
2014-02-27 19:40 ` Waiman Long
2014-02-26 15:14 ` [PATCH RFC v5 5/8] pvqspinlock, x86: Enable unfair queue spinlock in a KVM guest Waiman Long
2014-02-26 17:08 ` Konrad Rzeszutek Wilk
2014-02-28 17:08 ` Waiman Long
2014-02-27 9:41 ` Paolo Bonzini
2014-02-27 19:05 ` Waiman Long
2014-02-27 10:40 ` Raghavendra K T
2014-02-27 19:12 ` Waiman Long
2014-02-26 15:14 ` [PATCH RFC v5 6/8] pvqspinlock, x86: Rename paravirt_ticketlocks_enabled Waiman Long
2014-02-26 15:14 ` [PATCH RFC v5 7/8] pvqspinlock, x86: Add qspinlock para-virtualization support Waiman Long
2014-02-26 17:54 ` Konrad Rzeszutek Wilk
2014-02-27 12:11 ` David Vrabel
2014-02-27 13:11 ` Paolo Bonzini
2014-02-27 14:18 ` David Vrabel
2014-02-27 14:45 ` Paolo Bonzini
2014-02-27 15:22 ` Raghavendra K T
2014-02-27 15:50 ` Paolo Bonzini
2014-03-03 11:06 ` [Xen-devel] " David Vrabel
2014-02-27 20:50 ` Waiman Long
2014-02-27 19:42 ` Waiman Long
2014-02-26 15:14 ` [PATCH RFC v5 8/8] pvqspinlock, x86: Enable KVM to use qspinlock's PV support Waiman Long
2014-02-27 9:31 ` Paolo Bonzini
2014-02-27 18:36 ` Waiman Long
2014-02-26 17:00 ` [PATCH v5 0/8] qspinlock: a 4-byte queue spinlock with " Konrad Rzeszutek Wilk
2014-02-28 16:56 ` Waiman Long
2014-02-26 22:26 ` Paul E. McKenney
-- strict thread matches above, loose matches on Subject: below --
2014-02-27 4:32 Waiman Long
2014-02-27 4:32 ` [PATCH RFC v5 4/8] pvqspinlock, x86: Allow unfair spinlock in a real PV environment Waiman Long
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