ibmvnic: Only replenish rx pool when resources are getting low

Previously, the driver would replenish the rx pool if the polling
function consumed less than the budget. The logic being that the driver
did not exhaust its budget so that must mean that the driver is not busy
and has cycles to spare for replenishing the pool.

So pool replenishment happens on every poll which did not consume
the budget. This can very costly during request-response tests.

In fact, an extra ~100pps can be seen in TCP_RR_150 tests when we remove
this conditional. Trace results (ftrace, graph-time=1) for the poll
function are below:
Previous results:
    ibmvnic_poll = 64951846.0 us / 4167628.0 hits = AVG 15.58
    replenish_rx_pool = 17602846.0 us / 4710437.0 hits = AVG 3.74
Now:
    ibmvnic_poll = 57673941.0 us / 4791737.0 hits = AVG 12.04
    replenish_rx_pool = 3938171.6 us / 4314.0 hits = AVG 912.88

While the replenish function takes longer, it is hit less frequently
meaning the ibmvnic_poll function, on average, is faster.

Furthermore, this change does not have a negative effect on
performance bandwidth/latency measurements.

Signed-off-by: Nick Child <nnac123@linux.ibm.com>
Link: https://patch.msgid.link/20240807211809.1259563-2-nnac123@linux.ibm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/ibm/ibmvnic.c

@@ -3527,9 +3527,8 @@ static int ibmvnic_poll(struct napi_struct *napi, int budget)
 	}
 
 	if (adapter->state != VNIC_CLOSING &&
-	    ((atomic_read(&adapter->rx_pool[scrq_num].available) <
-	      adapter->req_rx_add_entries_per_subcrq / 2) ||
-	      frames_processed < budget))
+	    (atomic_read(&adapter->rx_pool[scrq_num].available) <
+	      adapter->req_rx_add_entries_per_subcrq / 2))
 		replenish_rx_pool(adapter, &adapter->rx_pool[scrq_num]);
 	if (frames_processed < budget) {
 		if (napi_complete_done(napi, frames_processed)) {