lkml.org 
[lkml]   [2019]   [Jul]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v3 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock
On Mon, Jul 15, 2019 at 03:25:34PM -0400, Alex Kogan wrote:
> +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;
> + struct cna_node *next = CNA_NODE(READ_ONCE(me->next));
> + int my_node;
> +
> + /* @next should be set, else we would not be calling this function. */
> + WARN_ON_ONCE(next == NULL);
> +
> + my_node = me_cna->numa_node;
> +
> + /*
> + * Fast path - check whether the immediate successor runs on
> + * the same node.
> + */
> + if (next->numa_node == my_node)
> + return next;
> +
> + head_other = next;
> + tail_other = next;
> +
> + /*
> + * Traverse the main waiting queue starting from the successor of my
> + * successor, and look for a thread running on the same node.
> + */
> + cur = CNA_NODE(READ_ONCE(next->mcs.next));
> + while (cur) {
> + if (cur->numa_node == my_node) {
> + /*
> + * Found a thread on the same node. Move threads
> + * between me and that node into the secondary queue.
> + */
> + if (me->locked > 1)
> + CNA_NODE(me->locked)->tail->mcs.next =
> + (struct mcs_spinlock *)head_other;
> + else
> + me->locked = (uintptr_t)head_other;
> + tail_other->mcs.next = NULL;
> + CNA_NODE(me->locked)->tail = tail_other;
> + return cur;
> + }
> + tail_other = cur;
> + cur = CNA_NODE(READ_ONCE(cur->mcs.next));
> + }
> + return NULL;
> +}

static void cna_move(struct cna_node *cn, struct cna_node *cni)
{
struct cna_node *head, *tail;

/* remove @cni */
WRITE_ONCE(cn->mcs.next, cni->mcs.next);

/* stick @cni on the 'other' list tail */
cni->mcs.next = NULL;

if (cn->mcs.locked <= 1) {
/* head = tail = cni */
head = cni;
head->tail = cni;
cn->mcs.locked = head->encoded_tail;
} else {
/* add to tail */
head = (struct cna_node *)decode_tail(cn->mcs.locked);
tail = tail->tail;
tail->next = cni;
}
}

static struct cna_node *cna_find_next(struct mcs_spinlock *node)
{
struct cna_node *cni, *cn = (struct cna_node *)node;

while ((cni = (struct cna_node *)READ_ONCE(cn->mcs.next))) {
if (likely(cni->node == cn->node))
break;

cna_move(cn, cni);
}

return cni;
}

> +static inline bool cna_set_locked_empty_mcs(struct qspinlock *lock, u32 val,
> + struct mcs_spinlock *node)
> +{
> + /* Check whether the secondary queue is empty. */
> + if (node->locked <= 1) {
> + if (atomic_try_cmpxchg_relaxed(&lock->val, &val,
> + _Q_LOCKED_VAL))
> + return true; /* No contention */
> + } else {
> + /*
> + * Pass the lock to the first thread in the secondary
> + * queue, but first try to update the queue's tail to
> + * point to the last node in the secondary queue.


That comment doesn't make sense; there's at least one conditional
missing.

> + */
> + struct cna_node *succ = CNA_NODE(node->locked);
> + u32 new = succ->tail->encoded_tail + _Q_LOCKED_VAL;
> +
> + if (atomic_try_cmpxchg_relaxed(&lock->val, &val, new)) {
> + arch_mcs_spin_unlock_contended(&succ->mcs.locked, 1);
> + return true;
> + }
> + }
> +
> + return false;
> +}

static cna_try_clear_tail(struct qspinlock *lock, u32 val, struct mcs_spinlock *node)
{
if (node->locked <= 1)
return __try_clear_tail(lock, val, node);

/* the other case */
}

> +static inline void cna_pass_mcs_lock(struct mcs_spinlock *node,
> + struct mcs_spinlock *next)
> +{
> + struct cna_node *succ = NULL;
> + u64 *var = &next->locked;
> + u64 val = 1;
> +
> + succ = find_successor(node);

This makes unlock O(n), which is 'funneh' and undocumented.

> +
> + if (succ) {
> + var = &succ->mcs.locked;
> + /*
> + * We unlock a successor by passing a non-zero value,
> + * so set @val to 1 iff @locked is 0, which will happen
> + * if we acquired the MCS lock when its queue was empty
> + */
> + val = node->locked + (node->locked == 0);
> + } else if (node->locked > 1) { /* if the secondary queue is not empty */
> + /* pass the lock to the first node in that queue */
> + succ = CNA_NODE(node->locked);
> + succ->tail->mcs.next = next;
> + var = &succ->mcs.locked;

> + } /*
> + * Otherwise, pass the lock to the immediate successor
> + * in the main queue.
> + */

I don't think this mis-indented comment can happen. The call-site
guarantees @next is non-null.

Therefore, cna_find_next() will either return it, or place it on the
secondary list. If it (cna_find_next) returns NULL, we must have a
non-empty secondary list.

In no case do I see this tertiary condition being possible.

> +
> + arch_mcs_spin_unlock_contended(var, val);
> +}

This also renders this @next argument superfluous.

static cna_mcs_pass_lock(struct mcs_spinlock *node, struct mcs_spinlock *next)
{
next = cna_find_next(node);
if (!next) {
BUG_ON(node->locked <= 1);
next = (struct cna_node *)decode_tail(node->locked);
node->locked = 1;
}

arch_mcs_pass_lock(&next->mcs.locked, node->locked);
}

\
 
 \ /
  Last update: 2019-07-16 17:52    [W:0.201 / U:22.056 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site