Do you get any zc notifications without IORING_NOTIF_USAGE_ZC_COPIED? It might be a reporting issue.

Do you get any zc notifications without IORING_NOTIF_USAGE_ZC_COPIED? It might be a reporting issue.

wow, some succeeded and some failed.
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When I restrict the use of IORING_OP_SEND_ZC/IORING_OP_SENDMSG_ZC to only buffers larger than 8k, all results are sendzc success.
Could it be that small buffers also fall back to copied?"

When I restrict the use of IORING_OP_SEND_ZC/IORING_OP_SENDMSG_ZC to only buffers larger than 8k, all results are sendzc success. Could it be that small buffers also fall back to copied?"

Nah, there is no fallback for TCP, it's up to the user. Considering the script above didn't print anything, I believe it is zerocopy, but the reporting is broken. Specifically for a performance feature called notification stacking.

Let's see if this bpftrace script will give anything interesting. You can run it as the previous one, but it'll likely be more convenient to put it into a file.

kprobe:io_tx_ubuf_complete {
	$skb = arg0;
	$uarg = (struct ubuf_info *)arg1;
	$success = arg2;
	$off = offsetof(struct io_notif_data, uarg);
	$nd = (struct io_notif_data *)((int8 *)$uarg - $off);
	$token = 0;
	if ($skb) {
		$token += 1;
	}
	if ($success) {
		$token += 10;
	}
	if ($nd->zc_report) {
		if ($nd->zc_used) {
			$token += 100;
		}
		if ($nd->zc_copied) {
			$token += 1000;
		}
	}
	if ((int64)$nd->next) {
		$token += 10000;
	}
	@[$token, kstack] = count();
}

You can also profile it, it should show if there were any copies:

perf record -g -- ./program
perf report --children --call-graph=graph,0 > file.txt