linux-kernel - Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to improve performance on some archs

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Date:   Thu, 10 Aug 2017 13:50:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Pan Xinhui <xinhui@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andrea Parri <parri.andrea@...il.com>
Subject: Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to
 improve performance on some archs

On Wed, May 24, 2017 at 09:38:28AM -0400, Waiman Long wrote:
> 
>   # of thread     w/o patch    with patch      % Change
>   -----------     ---------    ----------      --------
>        4         4053.3 Mop/s  4223.7 Mop/s     +4.2%
>        8         3310.4 Mop/s  3406.0 Mop/s     +2.9%
>       12         2576.4 Mop/s  2674.6 Mop/s     +3.8%

Waiman, could you run those numbers again but with the below 'fixed' ?

> @@ -361,6 +361,13 @@ static void pv_kick_node(struct qspinlock *lock, struct mcs_spinlock *node)
>  	 * observe its next->locked value and advance itself.
>  	 *
>  	 * Matches with smp_store_mb() and cmpxchg() in pv_wait_node()
> +	 *
> +	 * The write to next->locked in arch_mcs_spin_unlock_contended()
> +	 * must be ordered before the read of pn->state in the cmpxchg()
> +	 * below for the code to work correctly. However, this is not
> +	 * guaranteed on all architectures when the cmpxchg() call fails.
> +	 * Both x86 and PPC can provide that guarantee, but other
> +	 * architectures not necessarily.
>  	 */

	smp_mb();

>  	if (cmpxchg(&pn->state, vcpu_halted, vcpu_hashed) != vcpu_halted)
>  		return;

Ideally this Power CPU can optimize back-to-back SYNC instructions, but
who knows...