linux-kernel - Re: [PATCH 05/11] qspinlock: Optimize for smaller NR

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Date:	Mon, 7 Jul 2014 17:35:52 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Waiman.Long@...com, tglx@...utronix.de, mingo@...nel.org,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	paolo.bonzini@...il.com, konrad.wilk@...cle.com,
	boris.ostrovsky@...cle.com, paulmck@...ux.vnet.ibm.com,
	riel@...hat.com, torvalds@...ux-foundation.org,
	raghavendra.kt@...ux.vnet.ibm.com, david.vrabel@...rix.com,
	oleg@...hat.com, gleb@...hat.com, scott.norton@...com,
	chegu_vinod@...com
Subject: Re: [PATCH 05/11] qspinlock: Optimize for smaller NR_CPUS

On Mon, Jul 07, 2014 at 05:08:17PM +0200, Paolo Bonzini wrote:
> Il 07/07/2014 16:35, Peter Zijlstra ha scritto:
> >On Wed, Jun 18, 2014 at 01:39:52PM +0200, Paolo Bonzini wrote:
> >>Il 15/06/2014 14:47, Peter Zijlstra ha scritto:
> >>>
> >>>-	for (;;) {
> >>>-		new = (val & ~_Q_PENDING_MASK) | _Q_LOCKED_VAL;
> >>>-
> >>>-		old = atomic_cmpxchg(&lock->val, val, new);
> >>>-		if (old == val)
> >>>-			break;
> >>>-
> >>>-		val = old;
> >>>-	}
> >>>+	clear_pending_set_locked(lock, val);
> >>>	return;
> >>
> >>
> >>Might as well add clear_pending_set_locked already in patch 3.
> >
> >Patch 4, if anywhere.
> >
> 
> This code is added in patch 3:
> 
> +	/*
> +	 * take ownership and clear the pending bit.
> +	 *
> +	 * *,1,0 -> *,0,1
> +	 */
> +	for (;;) {
> +		new = (val & ~_Q_PENDING_MASK) | _Q_LOCKED_VAL;
> +
> +		old = atomic_cmpxchg(&lock->val, val, new);
> +		if (old == val)
> +			break;
> +
> +		val = old;
> +	}
> +	return;
> 
> Unlike the change in patch 4, clear_pending_set_locked doesn't change how
> qspinlock moves from a state to the next.

True, but its where we start to break up into smaller functions. And the
only reason we break them out is because we'll get different
implementations depending on NR_CPUS.

So we can view the breakout of xchg_tail and clear_and_set_pending as
preparatory work for introducing the NR_CPUS thing.

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