linux-kernel - Re: [PATCH v3 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock

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Date:   Tue, 16 Jul 2019 22:16:29 -0400
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, rahul.x.yadav@...cle.com
Subject: Re: [PATCH v3 3/5] locking/qspinlock: Introduce CNA into the slow
 path of qspinlock

On 7/15/19 3:25 PM, Alex Kogan wrote:
> +/*
> + * Implement a NUMA-aware version of MCS (aka CNA, or compact NUMA-aware lock).
> + *
> + * In CNA, spinning threads are organized in two queues, a main queue for
> + * threads running on the same node as the current lock holder, and a
> + * secondary queue for threads running on other nodes. At the unlock time,
> + * the lock holder scans the main queue looking for a thread running on
> + * the same node. If found (call it thread T), all threads in the main queue
> + * between the current lock holder and T are moved to the end of the
> + * secondary queue, and the lock is passed to T. If such T is not found, the
> + * lock is passed to the first node in the secondary queue. Finally, if the
> + * secondary queue is empty, the lock is passed to the next thread in the
> + * main queue. To avoid starvation of threads in the secondary queue,
> + * those threads are moved back to the head of the main queue
> + * after a certain expected number of intra-node lock hand-offs.
> + *
> + * For more details, see https://arxiv.org/abs/1810.05600.
> + *
> + * Authors: Alex Kogan <alex.kogan@...cle.com>
> + *          Dave Dice <dave.dice@...cle.com>
> + */
> +
> +struct cna_node {
> +	struct	mcs_spinlock mcs;
> +	u32	numa_node;
> +	u32	encoded_tail;
> +	struct	cna_node *tail;    /* points to the secondary queue tail */
> +};
> +
> +#define CNA_NODE(ptr) ((struct cna_node *)(ptr))
> +
> +static void cna_init_node(struct mcs_spinlock *node)
> +{
> +	struct cna_node *cn = CNA_NODE(node);
> +	struct mcs_spinlock *base_node;
> +	int cpuid;
> +
> +	BUILD_BUG_ON(sizeof(struct cna_node) > sizeof(struct qnode));
> +	/* we store a pointer in the node's @locked field */
> +	BUILD_BUG_ON(sizeof(uintptr_t) > sizeof_field(struct mcs_spinlock, locked));
> +
> +	cpuid = smp_processor_id();
> +	cn->numa_node = cpu_to_node(cpuid);
> +
> +	base_node = this_cpu_ptr(&qnodes[0].mcs);
> +	cn->encoded_tail = encode_tail(cpuid, base_node->count - 1);
> +}
> +
> +/**
> + * find_successor - Scan the main waiting queue looking for the first
> + * thread running on the same 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 T; otherwise, return NULL.
> + */

Here you talk about main and secondary queues. However, there is no
mention of what are those queues. As I am familiar with qspinlock queue,
I can figure out that the main queue is the mcs_node->next chain that
starts from the MCS lock holder. The secondary queue is a separate MCS
node chain with its head stored in the mcs_node->locked value of the MCS
lock holder and the tail stored in the cna->tail. More detail comments
on what and where they are will help to improve the readability of the
code. A simple graphic to illustrate those queues will help too, for example

/*
 * MCS lock holder
 * ===============
 *    mcs_node
 *   +--------+      +----+         +----+
 *   | next   | ---> |next| -> ...  |next| -> NULL  [Main queue]
 *   | locked | -+   +----+         +----+
 *   +--------+  |
 *               |   +----+         +----+
 *               +-> |next| -> ...  |next| -> X     [Secondary queue]
 *    cna_node       +----+         +----+
 *   +--------*                       ^
 *   | tail   | ----------------------+
 *   +--------*   
 *
 * N.B. locked = 1 if secondary queue is absent.
 */

> +static struct cna_node *find_successor(struct mcs_spinlock *me)
> +{
> +	struct cna_node *me_cna = CNA_NODE(me);
> +	struct cna_node *head_other, *tail_other, *cur;
As you have talked about secondary queue, how about head_2nd, tail_2nd
instead of *_other?

Cheers,
Longman