netdev - Re: [net-queue bugfix RFC] i40e: Clear IFF_RXFH

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Date:   Mon, 17 Oct 2022 12:45:55 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Joe Damato <jdamato@...tly.com>
Cc:     intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
        davem@...emloft.net, anthony.l.nguyen@...el.com,
        jesse.brandeburg@...el.com
Subject: Re: [net-queue bugfix RFC] i40e: Clear IFF_RXFH_CONFIGURED when RSS
 is reset

On Thu, 13 Oct 2022 15:54:31 -0700 Joe Damato wrote:
> Before this change, reconfiguring the queue count using ethtool doesn't
> always work, even for queue counts that were previously accepted because
> the IFF_RXFH_CONFIGURED bit was not cleared when the flow indirection hash
> is cleared by the driver.

It's not cleared but when was it set? Could you describe the flow that
gets us to this set a bit more?

Normally clearing the IFF_RXFH_CONFIGURED in the driver is _only_
acceptable on error recovery paths, and should come with a "this should
never happen" warning.

> For example:
> 
> $ sudo ethtool -x eth0
> RX flow hash indirection table for eth0 with 34 RX ring(s):
>     0:      0     1     2     3     4     5     6     7
>     8:      8     9    10    11    12    13    14    15
>    16:     16    17    18    19    20    21    22    23
>    24:     24    25    26    27    28    29    30    31
>    32:     32    33     0     1     2     3     4     5
> [...snip...]
> 
> As you can see, the flow indirection hash distributes flows to 34 queues.
> 
> Increasing the number of queues from 34 to 64 works, and the flow
> indirection hash is reset automatically:
> 
> $ sudo ethtool -L eth0 combined 64
> $ sudo ethtool -x eth0
> RX flow hash indirection table for eth0 with 64 RX ring(s):
>     0:      0     1     2     3     4     5     6     7
>     8:      8     9    10    11    12    13    14    15
>    16:     16    17    18    19    20    21    22    23
>    24:     24    25    26    27    28    29    30    31
>    32:     32    33    34    35    36    37    38    39
>    40:     40    41    42    43    44    45    46    47
>    48:     48    49    50    51    52    53    54    55
>    56:     56    57    58    59    60    61    62    63

This is odd, if IFF_RXFH_CONFIGURED is set driver should not
re-initialize the indirection table. Which I believe is what
you describe at the end of your message:

> But, I can increase the queue count and the flow hash is preserved:
> 
> $ sudo ethtool -L eth0 combined 64
> $ sudo ethtool -x eth0
> RX flow hash indirection table for eth0 with 64 RX ring(s):
>     0:      0     1     2     3     4     5     6     7
>     8:      8     9    10    11    12    13    14    15
>    16:     16    17    18    19     0     1     2     3