linux-kernel - Re: [PATCH RFC 1/2] qspinlock: Introducing a 4-byte queue spinlock implementation

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Date:	Thu, 1 Aug 2013 22:47:10 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Cc:	Waiman Long <Waiman.Long@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Richard Weinberger <richard@....at>,
	Catalin Marinas <catalin.marinas@....com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Matt Fleming <matt.fleming@...el.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Akinobu Mita <akinobu.mita@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Rik van Riel <riel@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	George Spelvin <linux@...izon.com>,
	Harvey Harrison <harvey.harrison@...il.com>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH RFC 1/2] qspinlock: Introducing a 4-byte queue spinlock
 implementation

On Fri, Aug 02, 2013 at 01:53:22AM +0530, Raghavendra K T wrote:

You need to learn to trim your replies.. I already stopped reading that
paravirt thread because of it. Soon I'll introduce you to my /dev/null
mail reader.

> On 08/01/2013 08:07 AM, Waiman Long wrote:
> >+static __always_inline void queue_spin_lock(struct qspinlock *lock)
> >+{
> >+	if (likely(queue_spin_trylock(lock)))
> >+		return;
> >+	queue_spin_lock_slowpath(lock);
> >+}
> 
> quickly falling into slowpath may hurt performance in some cases. no?
> 
> Instead, I tried something like this:
> 
> #define SPIN_THRESHOLD 64
> 
> static __always_inline void queue_spin_lock(struct qspinlock *lock)
> {
>         unsigned count = SPIN_THRESHOLD;
>         do {
>                 if (likely(queue_spin_trylock(lock)))
>                         return;
>                 cpu_relax();
>         } while (count--);
>         queue_spin_lock_slowpath(lock);
> }
> 
> Though I could see some gains in overcommit, but it hurted undercommit
> in some workloads :(.

This would break the FIFO nature of the lock.

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