netdev - Re: [PATCH v4] ixgbe: let the xdpdrv work with more than 64 cpus

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Date:   Thu, 26 Aug 2021 09:41:19 -0700
From:   Jesse Brandeburg <jesse.brandeburg@...el.com>
To:     Eric Dumazet <eric.dumazet@...il.com>, <kerneljasonxing@...il.com>,
        <anthony.l.nguyen@...el.com>, <davem@...emloft.net>,
        <kuba@...nel.org>, <ast@...nel.org>, <daniel@...earbox.net>,
        <hawk@...nel.org>, <john.fastabend@...il.com>, <andrii@...nel.org>,
        <kafai@...com>, <songliubraving@...com>, <yhs@...com>,
        <kpsingh@...nel.org>
CC:     <intel-wired-lan@...ts.osuosl.org>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <bpf@...r.kernel.org>,
        Jason Xing <xingwanli@...ishou.com>,
        Shujin Li <lishujin@...ishou.com>
Subject: Re: [PATCH v4] ixgbe: let the xdpdrv work with more than 64 cpus

On 8/26/2021 9:18 AM, Eric Dumazet wrote:

>> +static inline int ixgbe_determine_xdp_q_idx(int cpu)
>> +{
>> +	if (static_key_enabled(&ixgbe_xdp_locking_key))
>> +		return cpu % IXGBE_MAX_XDP_QS;
>> +	else
>> +		return cpu;
> 
> Even if num_online_cpus() is 8, the returned cpu here could be
> 
> 0, 32, 64, 96, 128, 161, 197, 224
> 
> Are we sure this will still be ok ?

I'm not sure about that one myself. Jason?

> 
>> +}
>> +
>>  static inline u8 ixgbe_max_rss_indices(struct ixgbe_adapter *adapter)
>>  {
>>  	switch (adapter->hw.mac.type) {
>> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c
>> index 0218f6c..884bf99 100644
>> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c
>> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c
>> @@ -299,7 +299,10 @@ static void ixgbe_cache_ring_register(struct ixgbe_adapter *adapter)
>>  
>>  static int ixgbe_xdp_queues(struct ixgbe_adapter *adapter)
>>  {
>> -	return adapter->xdp_prog ? nr_cpu_ids : 0;
>> +	int queues;
>> +
>> +	queues = min_t(int, IXGBE_MAX_XDP_QS, num_online_cpus());
> 
> num_online_cpus() might change later...

I saw that too, but I wonder if it doesn't matter to the driver. If a
CPU goes offline or comes online after the driver loads, we will use
this logic to try to pick an available TX queue. But this is a
complicated thing that is easy to get wrong, is there a common example
of how to get it right?

A possible problem I guess is that if the "static_key_enabled" check
returned false in the past, we would need to update that if the number
of CPUs changes, do we need a notifier?

Also, now that I'm asking it, I dislike the global as it would apply to
all ixgbe ports and each PF would increment and decrement it
independently. Showing my ignorance here, but I haven't seen this
utility in the kernel before in detail. Not sure if this is "OK" from
multiple device (with the same driver / global namespace) perspective.