linux-kernel - Re: [PATCH v8 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock

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Date:   Fri, 3 Jan 2020 17:14:55 -0500
From:   Waiman Long <longman@...hat.com>
To:     Alex Kogan <alex.kogan@...cle.com>, linux@...linux.org.uk,
        peterz@...radead.org, 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
Cc:     steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        dave.dice@...cle.com
Subject: Re: [PATCH v8 3/5] locking/qspinlock: Introduce CNA into the slow
 path of qspinlock

On 12/30/19 2:40 PM, Alex Kogan wrote:
> +/*
> + * cna_scan_main_queue - scan the main waiting queue looking for the first
> + * thread running on the same NUMA node as the lock holder. If found (call it
> + * thread T), move all threads in the main queue between the lock holder and
> + * T to the end of the secondary queue and return 0
> + * (=SUCCESSOR_FROM_SAME_NUMA_NODE_FOUND); otherwise, return the encoded
Are you talking about LOCAL_WAITER_FOUND?
> + * pointer of the last scanned node in the primary queue (so a subsequent scan
> + * can be resumed from that node).
> + *
> + * Schematically, this may look like the following (nn stands for numa_node and
> + * et stands for encoded_tail).
> + *
> + *   when cna_scan_main_queue() is called (the secondary queue is empty):
> + *
> + *  A+------------+   B+--------+   C+--------+   T+--------+
> + *   |mcs:next    | -> |mcs:next| -> |mcs:next| -> |mcs:next| -> NULL
> + *   |mcs:locked=1|    |cna:nn=0|    |cna:nn=2|    |cna:nn=1|
> + *   |cna:nn=1    |    +--------+    +--------+    +--------+
> + *   +----------- +
> + *
> + *   when cna_scan_main_queue() returns (the secondary queue contains B and C):
> + *
> + *  A+----------------+    T+--------+
> + *   |mcs:next        | ->  |mcs:next| -> NULL
> + *   |mcs:locked=C.et | -+  |cna:nn=1|
> + *   |cna:nn=1        |  |  +--------+
> + *   +--------------- +  +-----+
> + *                             \/
> + *          B+--------+   C+--------+
> + *           |mcs:next| -> |mcs:next| -+
> + *           |cna:nn=0|    |cna:nn=2|  |
> + *           +--------+    +--------+  |
> + *               ^                     |
> + *               +---------------------+
> + *
> + * The worst case complexity of the scan is O(n), where n is the number
> + * of current waiters. However, the amortized complexity is close to O(1),
> + * as the immediate successor is likely to be running on the same node once
> + * threads from other nodes are moved to the secondary queue.
> + *
> + * @node      : Pointer to the MCS node of the lock holder
> + * @pred_start: Pointer to the MCS node of the waiter whose successor should be
> + *              the first node in the scan
> + * Return     : LOCAL_WAITER_FOUND or encoded tail of the last scanned waiter
> + */
> +static u32 cna_scan_main_queue(struct mcs_spinlock *node,
> +			       struct mcs_spinlock *pred_start)
> +{
> +	struct cna_node *cn = (struct cna_node *)node;
> +	struct cna_node *cni = (struct cna_node *)READ_ONCE(pred_start->next);
> +	struct cna_node *last;
> +	int my_numa_node = cn->numa_node;
> +
> +	/* find any next waiter on 'our' NUMA node */
> +	for (last = cn;
> +	     cni && cni->numa_node != my_numa_node;
> +	     last = cni, cni = (struct cna_node *)READ_ONCE(cni->mcs.next))
> +		;
> +
> +	/* if found, splice any skipped waiters onto the secondary queue */
> +	if (cni) {
> +		if (last != cn)	/* did we skip any waiters? */
> +			cna_splice_tail(node, node->next,
> +					(struct mcs_spinlock *)last);
> +		return LOCAL_WAITER_FOUND;
> +	}
> +
> +	return last->encoded_tail;
> +}
> +
>
> +/*
> + * 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 cna_configure_spin_lock_slowpath(void)
Nit: There should be a __init.
> +{
> +	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");
> +	}
> +}

Other than these two minor nits, the rests looks good to me.

Cheers,
Longman