A lot of CPU time is spent simply locking/unlocking q->lock during
polling. Check for completion outside the lock to make q->lock disappear
from the profile.
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Sergio Lopez <slp@redhat.com>
Message-id:
20200617132201.
1832152-2-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
for (i = 0; i < s->nr_queues; i++) {
NVMeQueuePair *q = s->queues[i];
+ const size_t cqe_offset = q->cq.head * NVME_CQ_ENTRY_BYTES;
+ NvmeCqe *cqe = (NvmeCqe *)&q->cq.queue[cqe_offset];
+
+ /*
+ * Do an early check for completions. q->lock isn't needed because
+ * nvme_process_completion() only runs in the event loop thread and
+ * cannot race with itself.
+ */
+ if ((le16_to_cpu(cqe->status) & 0x1) == q->cq_phase) {
+ continue;
+ }
+
qemu_mutex_lock(&q->lock);
while (nvme_process_completion(s, q)) {
/* Keep polling */