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From: Waiman Long <waiman.long@hp.com>
To: David Vrabel <david.vrabel@citrix.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>,
	Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>,
	kvm@vger.kernel.org, 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>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-arch@vger.kernel.org, Gleb Natapov <gleb@redhat.com>,
	x86@kernel.org, Ingo Molnar <mingo@redhat.com>,
	xen-devel@lists.xenproject.org,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Rik van Riel <riel@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Scott J Norton <scott.norton@hp.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Chris Wright <chrisw@sous-sol.org>,
	Oleg Nesterov <oleg@redhat.com>,
	Alok Kataria <akataria@vmware.com>,
	Aswin Chandramouleeswaran <aswin@hp.com>, Chegu Vinod <chegu>
Subject: Re: [PATCH RFC v6 09/11] pvqspinlock, x86: Add qspinlock para-virtualization support
Date: Thu, 13 Mar 2014 15:05:57 -0400	[thread overview]
Message-ID: <53220195.6090108@hp.com> (raw)
In-Reply-To: <5321949F.1010103@citrix.com>

On 03/13/2014 07:21 AM, David Vrabel wrote:
> On 12/03/14 18:54, Waiman Long wrote:
>> This patch adds para-virtualization support to the queue spinlock in
>> the same way as was done in the PV ticket lock code. In essence, the
>> lock waiters will spin for a specified number of times (QSPIN_THRESHOLD
>> = 2^14) and then halted itself. The queue head waiter will spins
>> 2*QSPIN_THRESHOLD times before halting itself. When it has spinned
>> QSPIN_THRESHOLD times, the queue head will assume that the lock
>> holder may be scheduled out and attempt to kick the lock holder CPU
>> if it has the CPU number on hand.
> I don't really understand the reasoning for kicking the lock holder.  It
> will either be: running, runnable, or halted because it's in a slow path
> wait for another lock.  In any of these states I do not see how a kick
> is useful.

You may be right. I can certainly take this part out of the patch if 
people don't think that is useful.

>> Enabling the PV code does have a performance impact on spinlock
>> acquisitions and releases. The following table shows the execution
>> time (in ms) of a spinlock micro-benchmark that does lock/unlock
>> operations 5M times for each task versus the number of contending
>> tasks on a Westmere-EX system.
>>
>>    # of        Ticket lock	     Queue lock
>>    tasks   PV off/PV on/%Change 	  PV off/PV on/%Change
>>    ------  --------------------   ---------------------
>>      1	     135/  179/+33%	     137/  169/+23%
>>      2	    1045/ 1103/ +6%	    1120/ 1536/+37%
>>      3	    1827/ 2683/+47%	    2313/ 2425/ +5%
>>      4       2689/ 4191/+56%	    2914/ 3128/ +7%
>>      5       3736/ 5830/+56%	    3715/ 3762/ +1%
>>      6       4942/ 7609/+54%	    4504/ 4558/ +2%
>>      7       6304/ 9570/+52%	    5292/ 5351/ +1%
>>      8       7736/11323/+46%	    6037/ 6097/ +1%
> Do you have measurements from tests when VCPUs are overcommitted?

I don't have a measurement with overcommitted guests yet. I will set up 
such an environment and do some tests on it.

>> +#ifdef CONFIG_PARAVIRT_SPINLOCKS
>> +/**
>> + * queue_spin_unlock_slowpath - kick up the CPU of the queue head
>> + * @lock : Pointer to queue spinlock structure
>> + *
>> + * The lock is released after finding the queue head to avoid racing
>> + * condition between the queue head and the lock holder.
>> + */
>> +void queue_spin_unlock_slowpath(struct qspinlock *lock)
>> +{
>> +	struct qnode *node, *prev;
>> +	u32 qcode = (u32)queue_get_qcode(lock);
>> +
>> +	/*
>> +	 * Get the queue tail node
>> +	 */
>> +	node = xlate_qcode(qcode);
>> +
>> +	/*
>> +	 * Locate the queue head node by following the prev pointer from
>> +	 * tail to head.
>> +	 * It is assumed that the PV guests won't have that many CPUs so
>> +	 * that it won't take a long time to follow the pointers.
> This isn't a valid assumption, but this isn't that different from the
> search done in the ticket slow unlock path so I guess it's ok.
>
> David

I will change that to say that in most cases, the queue length will be 
short.

-Longman

  parent reply	other threads:[~2014-03-13 19:05 UTC|newest]

Thread overview: 59+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-03-12 18:54 [PATCH v6 00/11] qspinlock: a 4-byte queue spinlock with PV support Waiman Long
2014-03-12 18:54 ` [PATCH v6 01/11] qspinlock: A generic 4-byte queue spinlock implementation Waiman Long
2014-03-12 18:54 ` [PATCH v6 02/11] qspinlock, x86: Enable x86-64 to use queue spinlock Waiman Long
2014-03-12 18:54 ` [PATCH v6 03/11] qspinlock: More optimized code for smaller NR_CPUS Waiman Long
2014-03-12 18:54 ` [PATCH v6 04/11] qspinlock: Optimized code path for 2 contending tasks Waiman Long
2014-03-12 19:08   ` Waiman Long
2014-03-13 13:57     ` Peter Zijlstra
2014-03-17 17:23       ` Waiman Long
2014-03-12 18:54 ` [PATCH v6 05/11] pvqspinlock, x86: Allow unfair spinlock in a PV guest Waiman Long
2014-03-13 10:54   ` David Vrabel
2014-03-13 13:16     ` Paolo Bonzini
2014-03-13 13:16       ` Paolo Bonzini
2014-03-17 19:05       ` Konrad Rzeszutek Wilk
2014-03-17 19:05         ` Konrad Rzeszutek Wilk
2014-03-18  8:14         ` Paolo Bonzini
2014-03-18  8:14           ` Paolo Bonzini
2014-03-19  3:15           ` Waiman Long
2014-03-19  3:15             ` Waiman Long
2014-03-19 10:07             ` Paolo Bonzini
2014-03-19 16:58               ` Waiman Long
2014-03-19 17:08                 ` Paolo Bonzini
2014-03-19 17:08                   ` Paolo Bonzini
2014-03-13 19:03     ` Waiman Long
2014-03-13 15:15   ` Peter Zijlstra
2014-03-13 20:05     ` Waiman Long
2014-03-14  8:30       ` Peter Zijlstra
2014-03-14  8:48         ` Paolo Bonzini
2014-03-14  8:48           ` Paolo Bonzini
2014-03-17 17:44         ` Waiman Long
2014-03-17 18:54           ` Peter Zijlstra
2014-03-18  8:16             ` Paolo Bonzini
2014-03-18  8:16               ` Paolo Bonzini
2014-03-19  3:08             ` Waiman Long
2014-03-17 19:10           ` Konrad Rzeszutek Wilk
2014-03-19  3:11             ` Waiman Long
2014-03-19 15:25               ` Konrad Rzeszutek Wilk
2014-03-12 18:54 ` [PATCH v6 06/11] pvqspinlock, x86: Allow unfair queue spinlock in a KVM guest Waiman Long
2014-03-12 18:54 ` [PATCH v6 07/11] pvqspinlock, x86: Allow unfair queue spinlock in a XEN guest Waiman Long
2014-03-12 18:54 ` [PATCH v6 08/11] pvqspinlock, x86: Rename paravirt_ticketlocks_enabled Waiman Long
2014-03-12 18:54 ` [PATCH RFC v6 09/11] pvqspinlock, x86: Add qspinlock para-virtualization support Waiman Long
2014-03-13 11:21   ` David Vrabel
2014-03-13 13:57     ` Paolo Bonzini
2014-03-13 13:57       ` Paolo Bonzini
2014-03-13 19:49       ` Waiman Long
2014-03-13 19:49         ` Waiman Long
2014-03-14  9:44         ` Paolo Bonzini
2014-03-14  9:44           ` Paolo Bonzini
2014-03-13 19:05     ` Waiman Long [this message]
2014-03-12 18:54 ` [PATCH RFC v6 10/11] pvqspinlock, x86: Enable qspinlock PV support for KVM Waiman Long
2014-03-13 13:59   ` Paolo Bonzini
2014-03-13 13:59     ` Paolo Bonzini
2014-03-13 19:13     ` Waiman Long
2014-03-14  8:42       ` Paolo Bonzini
2014-03-17 17:47         ` Waiman Long
2014-03-17 17:47           ` Waiman Long
2014-03-18  8:18           ` Paolo Bonzini
2014-03-13 15:25   ` Peter Zijlstra
2014-03-13 20:09     ` Waiman Long
2014-03-12 18:54 ` [PATCH RFC v6 11/11] pvqspinlock, x86: Enable qspinlock PV support for XEN Waiman Long

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