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

Alex Kogan
2020-01-06T15:04:27Z

> On Jan 3, 2020, at 5:14 PM, Waiman Long <longman@redhat.com> wrote:
>
> 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?
Ahh, yes — good catch!

>> + * 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.
True. I will fix that.

>> +{
>> + 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.
Great. I will revise and resubmit.

Best regards,
— Alex