linux-kernel - Re: [PATCH v9 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock

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Date:   Thu, 23 Jan 2020 16:29:18 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Alex Kogan <alex.kogan@...cle.com>, linux@...linux.org.uk,
        mingo@...hat.com, will.deacon@....com, arnd@...db.de,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, bp@...en8.de,
        hpa@...or.com, x86@...nel.org, guohanjun@...wei.com,
        jglauber@...vell.com, steven.sistare@...cle.com,
        daniel.m.jordan@...cle.com, dave.dice@...cle.com
Subject: Re: [PATCH v9 3/5] locking/qspinlock: Introduce CNA into the slow
 path of qspinlock

On Thu, Jan 23, 2020 at 09:15:55AM -0500, Waiman Long wrote:
> On 1/14/20 10:59 PM, Alex Kogan wrote:
> > +static int __init cna_init_nodes(void)
> > +{
> > +	unsigned int cpu;
> > +
> > +	/*
> > +	 * this will break on 32bit architectures, so we restrict
> > +	 * the use of CNA to 64bit only (see arch/x86/Kconfig)
> > +	 */
> > +	BUILD_BUG_ON(sizeof(struct cna_node) > sizeof(struct qnode));
> > +	/* we store an ecoded tail word in the node's @locked field */
> > +	BUILD_BUG_ON(sizeof(u32) > sizeof(unsigned int));
> > +
> > +	for_each_possible_cpu(cpu)
> > +		cna_init_nodes_per_cpu(cpu);
> > +
> > +	return 0;
> > +}
> > +early_initcall(cna_init_nodes);
> > +
> 
> I just realized that you shouldn't call cna_init_nodes as an
> early_initcall. Instead,
> 
> > +/*
> > + * Switch to the NUMA-friendly slow path for spinlocks when we have
> > + * multiple NUMA nodes in native environment, unless the user has
> > + * overridden this default behavior by setting the numa_spinlock flag.
> > + */
> > +void __init cna_configure_spin_lock_slowpath(void)
> > +{
> > +	if ((numa_spinlock_flag == 1) ||
> > +	    (numa_spinlock_flag == 0 && nr_node_ids > 1 &&
> > +		    pv_ops.lock.queued_spin_lock_slowpath ==
> > +			native_queued_spin_lock_slowpath)) {
> > +		pv_ops.lock.queued_spin_lock_slowpath =
> > +		    __cna_queued_spin_lock_slowpath;
> > +
> > +		pr_info("Enabling CNA spinlock\n");
> > +	}
> > +}
> 
> call it when it is sure that CNA spinlock is going to be used. At this
> point, the system is still in UP mode and the slowpath will not be called.

Indeed, let me go fix that!