From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <Waiman.Long@hp.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
linux-arch@vger.kernel.org, x86@kernel.org,
linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org,
xen-devel@lists.xenproject.org, kvm@vger.kernel.org,
Paolo Bonzini <paolo.bonzini@gmail.com>,
Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
Boris Ostrovsky <boris.ostrovsky@oracle.com>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Rik van Riel <riel@redhat.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>,
David Vrabel <david.vrabel@citrix.com>,
Oleg Nesterov <oleg@redhat.com>, Gleb Natapov <gleb@redhat.com>,
Scott J Norton <scott.norton@hp.com>,
Chegu Vinod <chegu_vinod@hp.com>
Subject: Re: [PATCH v11 09/16] qspinlock, x86: Allow unfair spinlock in a virtual guest
Date: Wed, 11 Jun 2014 13:38:41 +0200 [thread overview]
Message-ID: <20140611113841.GD3588@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20140611105402.GL3213@twins.programming.kicks-ass.net>
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On Wed, Jun 11, 2014 at 12:54:02PM +0200, Peter Zijlstra wrote:
> > @@ -252,6 +260,18 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> >
> > BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
> >
> > +#ifdef CONFIG_VIRT_UNFAIR_LOCKS
> > + /*
> > + * A simple test and set unfair lock
> > + */
> > + if (static_key_false(&virt_unfairlocks_enabled)) {
> > + cpu_relax(); /* Relax after a failed lock attempt */
>
> Meh, I don't think anybody can tell the difference if you put that in or
> not, therefore don't.
>
> > + while (!queue_spin_trylock(lock))
> > + cpu_relax();
> > + return;
> > + }
> > +#endif /* CONFIG_VIRT_UNFAIR_LOCKS */
>
> If you're really worried about those upper 24bits, you can always clear
> them when you get here.
I don't think its a problem at all; flipping the static_key requires
stop_machine, which guarantees us that there are no spinlocks held. So I
think you can actually BUG_ON() the upper 24bits.
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next prev parent reply other threads:[~2014-06-11 11:38 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-05-30 15:43 [PATCH v11 00/16] qspinlock: a 4-byte queue spinlock with PV support Waiman Long
2014-05-30 15:43 ` [PATCH v11 01/16] qspinlock: A simple generic 4-byte queue spinlock Waiman Long
2014-05-30 15:43 ` [PATCH v11 02/16] qspinlock, x86: Enable x86-64 to use " Waiman Long
2014-05-30 15:43 ` [PATCH v11 03/16] qspinlock: Add pending bit Waiman Long
2014-05-30 15:43 ` [PATCH v11 04/16] qspinlock: Extract out the exchange of tail code word Waiman Long
2014-05-30 15:43 ` [PATCH v11 05/16] qspinlock: Optimize for smaller NR_CPUS Waiman Long
2014-05-30 15:43 ` [PATCH v11 06/16] qspinlock: prolong the stay in the pending bit path Waiman Long
2014-06-11 10:26 ` Peter Zijlstra
2014-06-11 21:22 ` Long, Wai Man
2014-06-12 6:00 ` Peter Zijlstra
2014-06-12 20:54 ` Waiman Long
2014-06-15 13:12 ` Peter Zijlstra
2014-05-30 15:43 ` [PATCH v11 07/16] qspinlock: Use a simple write to grab the lock, if applicable Waiman Long
2014-05-30 15:43 ` [PATCH v11 08/16] qspinlock: Prepare for unfair lock support Waiman Long
2014-05-30 15:43 ` [PATCH v11 09/16] qspinlock, x86: Allow unfair spinlock in a virtual guest Waiman Long
2014-06-11 10:54 ` Peter Zijlstra
2014-06-11 11:38 ` Peter Zijlstra [this message]
2014-06-12 1:37 ` Long, Wai Man
2014-06-12 5:50 ` Peter Zijlstra
2014-06-12 21:08 ` Waiman Long
2014-06-15 13:14 ` Peter Zijlstra
2014-05-30 15:43 ` [PATCH v11 10/16] qspinlock: Split the MCS queuing code into a separate slowerpath Waiman Long
2014-05-30 15:43 ` [PATCH v11 11/16] pvqspinlock, x86: Rename paravirt_ticketlocks_enabled Waiman Long
2014-05-30 15:43 ` [PATCH v11 12/16] pvqspinlock, x86: Add PV data structure & methods Waiman Long
2014-05-30 15:43 ` [PATCH v11 13/16] pvqspinlock: Enable coexistence with the unfair lock Waiman Long
2014-05-30 15:44 ` [PATCH v11 14/16] pvqspinlock: Add qspinlock para-virtualization support Waiman Long
2014-06-12 8:17 ` Peter Zijlstra
2014-06-12 20:48 ` Waiman Long
2014-06-15 13:16 ` Peter Zijlstra
2014-06-17 20:59 ` Konrad Rzeszutek Wilk
2014-05-30 15:44 ` [PATCH v11 15/16] pvqspinlock, x86: Enable PV qspinlock PV for KVM Waiman Long
2014-05-30 15:44 ` [PATCH v11 16/16] pvqspinlock, x86: Enable PV qspinlock for XEN Waiman Long
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