On Wed, Jun 17, 2020 at 02:21:55PM +0100, Stefan Hajnoczi wrote:
> 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 <stefa...@redhat.com>
> ---
> block/nvme.c | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
> diff --git a/block/nvme.c b/block/nvme.c
> index eb2f54dd9d..e4375ec245 100644
> --- a/block/nvme.c
> +++ b/block/nvme.c
> @@ -512,6 +512,18 @@ static bool nvme_poll_queues(BDRVNVMeState *s)
>
> 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 */
> --
> 2.26.2
>Thanks for extending the comment. Reviewed-by: Sergio Lopez <s...@redhat.com>
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