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

Waiman Long
2020-01-03T22:14:55Z

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