netdev - Re: [Intel-wired-lan] [PATCH iwl-next v3] ixgbe: preserve RSS indirection table across admin down/up

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Message-ID: <20250902210447.77961-1-enjuk@amazon.com>
Date: Wed, 3 Sep 2025 06:04:42 +0900
From: Kohei Enju <enjuk@...zon.com>
To: <aleksandr.loktionov@...el.com>
CC: <andrew+netdev@...n.ch>, <anthony.l.nguyen@...el.com>,
	<davem@...emloft.net>, <edumazet@...gle.com>, <enjuk@...zon.com>,
	<intel-wired-lan@...ts.osuosl.org>, <kohei.enju@...il.com>,
	<kuba@...nel.org>, <netdev@...r.kernel.org>, <pabeni@...hat.com>,
	<przemyslaw.kitszel@...el.com>
Subject: Re: [Intel-wired-lan] [PATCH iwl-next v3] ixgbe: preserve RSS indirection table across admin down/up

On Tue, 2 Sep 2025 13:25:56 +0000, Loktionov, Aleksandr wrote:

> [...]
>> -
>> -	for (i = 0, j = 0; i < reta_entries; i++, j++) {
>> -		if (j == rss_i)
>> -			j = 0;
>> +	/* Update redirection table in memory on first init, queue
>> count change,
>> +	 * or reta entries change, otherwise preserve user
>> configurations. Then
>> +	 * always write to hardware.
>> +	 */
>> +	if (adapter->last_rss_indices != rss_i ||
>> +	    adapter->last_reta_entries != reta_entries) {
>> +		for (i = 0; i < reta_entries; i++)
>> +			adapter->rss_indir_tbl[i] = i % rss_i;
>Are you sure rss_i never ever can be a 0?
>This is the only thing I'm worrying about.

Oops, you're exactly right. Good catch!

I see the original code assigns 0 to rss_indir_tbl[i] when rss_i is 0,
like:
  adapter->rss_indir_tbl[i] = 0;

To handle this with keeping the behavior when rss_i == 0, I'm
considering
Option 1:
  adapter->rss_indir_tbl[i] = rss_i ? i % rss_i : 0;

Option 2:
  if (rss_i)
      for (i = 0; i < reta_entries; i++)
          adapter->rss_indir_tbl[i] = i % rss_i;
  else
      memset(adapter->rss_indir_tbl, 0, reta_entries);

Since this is not in the data path, the overhead of checking rss_i in
each iteration might be acceptable. Therefore I'd like to adopt the
option 1 for simplicity.

Do you have any preference or other suggestions?