Re: [PATCH net-next v8 11/15] ibmveth: Add per-queue RX and TX statistics collection

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- [Medium] Statistics readers now take every RX queue's replenish_lock
  with spin_lock_bh().

--

ibmveth: Add per-queue RX and TX statistics collection

This patch moves the adapter-wide RX and TX counters into per-queue
ibmveth_rx_queue_stats and ibmveth_tx_queue_stats arrays allocated at
probe. It exports packets, bytes and hardware drops through
netdev_stat_ops, adds ndo_get_stats64(), and keeps the existing ethtool -S
keys by summing the per-queue slots. It also adds a per-queue
no_buffer_retired carry so that rx_no_buffer never goes backwards across a
reopen.

> diff --git a/drivers/net/ethernet/ibm/ibmveth.c b/drivers/net/ethernet/ibm/ibmveth.c
> index a0c3c06d2b9d9..45adf6e9be09b 100644
> --- a/drivers/net/ethernet/ibm/ibmveth.c
> +++ b/drivers/net/ethernet/ibm/ibmveth.c

[ ... ]

> @@ -1063,7 +1197,45 @@ static void ibmveth_update_rx_no_buffer(struct ibmveth_adapter *adapter,
>  	p = adapter->buffer_list_addr[queue_index] + 4096 - 8;
>  	drops = be64_to_cpup(p);
>  
> -	adapter->rx_no_buffer = drops;
> +	qstats = &adapter->rx_qstats[queue_index];
> +
> +	if (drops < qstats->no_buffer_drops)
> +		IBMVETH_STAT_ADD(qstats->no_buffer_retired,
> +				 qstats->no_buffer_drops);
> +	WRITE_ONCE(qstats->no_buffer_drops, drops);
> +}
> +
> +/* Snapshot live+retired under replenish_lock; see retire_rx_no_buffer(). */
> +static u64 ibmveth_rx_no_buffer_pair(struct ibmveth_adapter *adapter, int i)
> +{
> +	struct ibmveth_rx_q *rxq = &adapter->rx_queue[i];
> +	struct ibmveth_rx_queue_stats *qstats = &adapter->rx_qstats[i];
> +	u64 drops, retired;
> +
> +	spin_lock_bh(&rxq->replenish_lock);
> +	drops = qstats->no_buffer_drops;
> +	retired = qstats->no_buffer_retired;
> +	spin_unlock_bh(&rxq->replenish_lock);
> +	return drops + retired;
> +}

[Severity: Medium]
Should the stats readers be taking replenish_lock here?

This lock sits on the RX datapath. ibmveth_remove_buffer_from_pool() takes
it from NAPI for every received frame. ibmveth_replenish_task() holds it
for a whole multi-pool refill, which includes netdev_alloc_skb(),
dma_map_single_attrs() and the h_add_logical_lan_buffers*() hcalls.

The new ndo_get_stats64 calls this helper for every slot, whether the
queue is live or not:

ibmveth_get_stats64() {
	for (i = 0; i < IBMVETH_MAX_RX_QUEUES; i++) {
		...
		stats->rx_missed_errors +=
			ibmveth_rx_no_buffer_pair(adapter, i);
	}
}

The rx_no_buffer ethtool -S key, ibmveth_get_queue_stats_rx() and
ibmveth_get_base_stats() also call it.

As a result, every /proc/net/dev read, RTM_GETLINK dump or bonding/team
stats poll takes up to 16 BH-disabling spinlocks. Unprivileged users can
trigger these reads.

Each acquisition pulls the lock cacheline away from the NAPI CPUs. A
reader can also spin with BH disabled for as long as a refill takes, for
example during the initial replenish in ibmveth_open().

Before this patch the driver had no ndo_get_stats64, and netdev->stats was
read without taking any lock.

Writers already hold replenish_lock. Could they keep a single combined
monotonic value (retired plus live drops) updated with WRITE_ONCE()? Then
readers could use READ_ONCE() and skip the lock.

This also looks unchanged at the end of the series, in "ibmveth: Complete
set_channels down-path". There, ibmveth_rx_no_buffer_pair() still uses
spin_lock_bh() and ibmveth_get_stats64() still calls it for every slot.

[ ... ]

-- 
Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261109235556.60604-1-mmc%40linux.ibm.com