There are three issues with the current NVMeRequest->busy field:
1. The busy field is accidentally accessed outside q->lock when request
submission fails.
2. Waiters on free_req_queue are not woken when a request is returned
early due to submission failure.
2. Finding a free request involves scanning all requests. This makes
request submission O(n^2).
Switch to an O(1) freelist that is always accessed under the lock.
Also differentiate between NVME_QUEUE_SIZE, the actual SQ/CQ size, and
NVME_NUM_REQS, the number of usable requests. This makes the code
simpler than using NVME_QUEUE_SIZE everywhere and having to keep in mind
that one slot is reserved.
Signed-off-by: Stefan Hajnoczi
---
block/nvme.c | 81 ++--
1 file changed, 54 insertions(+), 27 deletions(-)
diff --git a/block/nvme.c b/block/nvme.c
index 6bf58bc6aa..3ad4f27e1c 100644
--- a/block/nvme.c
+++ b/block/nvme.c
@@ -33,6 +33,12 @@
#define NVME_QUEUE_SIZE 128
#define NVME_BAR_SIZE 8192
+/*
+ * We have to leave one slot empty as that is the full queue case where
+ * head == tail + 1.
+ */
+#define NVME_NUM_REQS (NVME_QUEUE_SIZE - 1)
+
typedef struct {
int32_t head, tail;
uint8_t *queue;
@@ -47,7 +53,7 @@ typedef struct {
int cid;
void *prp_list_page;
uint64_t prp_list_iova;
-bool busy;
+int free_req_next; /* q->reqs[] index of next free req */
} NVMeRequest;
typedef struct {
@@ -61,7 +67,8 @@ typedef struct {
/* Fields protected by @lock */
NVMeQueue sq, cq;
int cq_phase;
-NVMeRequest reqs[NVME_QUEUE_SIZE];
+int free_req_head;
+NVMeRequest reqs[NVME_NUM_REQS];
boolbusy;
int need_kick;
int inflight;
@@ -200,19 +207,23 @@ static NVMeQueuePair
*nvme_create_queue_pair(BlockDriverState *bs,
qemu_mutex_init(>lock);
q->index = idx;
qemu_co_queue_init(>free_req_queue);
-q->prp_list_pages = qemu_blockalign0(bs, s->page_size * NVME_QUEUE_SIZE);
+q->prp_list_pages = qemu_blockalign0(bs, s->page_size * NVME_NUM_REQS);
r = qemu_vfio_dma_map(s->vfio, q->prp_list_pages,
- s->page_size * NVME_QUEUE_SIZE,
+ s->page_size * NVME_NUM_REQS,
false, _list_iova);
if (r) {
goto fail;
}
-for (i = 0; i < NVME_QUEUE_SIZE; i++) {
+q->free_req_head = -1;
+for (i = 0; i < NVME_NUM_REQS; i++) {
NVMeRequest *req = >reqs[i];
req->cid = i + 1;
+req->free_req_next = q->free_req_head;
+q->free_req_head = i;
req->prp_list_page = q->prp_list_pages + i * s->page_size;
req->prp_list_iova = prp_list_iova + i * s->page_size;
}
+
nvme_init_queue(bs, >sq, size, NVME_SQ_ENTRY_BYTES, _err);
if (local_err) {
error_propagate(errp, local_err);
@@ -254,13 +265,11 @@ static void nvme_kick(BDRVNVMeState *s, NVMeQueuePair *q)
*/
static NVMeRequest *nvme_get_free_req(NVMeQueuePair *q)
{
-int i;
-NVMeRequest *req = NULL;
+NVMeRequest *req;
qemu_mutex_lock(>lock);
-while (q->inflight + q->need_kick > NVME_QUEUE_SIZE - 2) {
-/* We have to leave one slot empty as that is the full queue case (head
- * == tail + 1). */
+
+while (q->free_req_head == -1) {
if (qemu_in_coroutine()) {
trace_nvme_free_req_queue_wait(q);
qemu_co_queue_wait(>free_req_queue, >lock);
@@ -269,20 +278,41 @@ static NVMeRequest *nvme_get_free_req(NVMeQueuePair *q)
return NULL;
}
}
-for (i = 0; i < NVME_QUEUE_SIZE; i++) {
-if (!q->reqs[i].busy) {
-q->reqs[i].busy = true;
-req = >reqs[i];
-break;
-}
-}
-/* We have checked inflight and need_kick while holding q->lock, so one
- * free req must be available. */
-assert(req);
+
+req = >reqs[q->free_req_head];
+q->free_req_head = req->free_req_next;
+req->free_req_next = -1;
+
qemu_mutex_unlock(>lock);
return req;
}
+/* With q->lock */
+static void nvme_put_free_req_locked(NVMeQueuePair *q, NVMeRequest *req)
+{
+req->free_req_next = q->free_req_head;
+q->free_req_head = req - q->reqs;
+}
+
+/* With q->lock */
+static void nvme_wake_free_req_locked(BDRVNVMeState *s, NVMeQueuePair *q)
+{
+if (!qemu_co_queue_empty(>free_req_queue)) {
+replay_bh_schedule_oneshot_event(s->aio_context,
+nvme_free_req_queue_cb, q);
+}
+}
+
+/* Insert a request in the freelist and wake waiters */
+static void nvme_put_free_req_and_wake(BDRVNVMeState *s, NVMeQueuePair *q,
+ NVMeRequest *req)
+{
+qemu_mutex_lock(>lock);
+nvme_put_free_req_locked(q, req);
+nvme_wake_free_req_locked(s, q);
+qemu_mutex_unlock(>lock);
+}
+
static inline int nvme_translate_error(const NvmeCqe *c)
{
uint16_t status = (le16_to_cpu(c->status) >> 1)
