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From:   Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@kernel.org>
To:     Alexander Wetzel <alexander@wetzel-home.de>,
        Felix Fietkau <nbd@nbd.name>,
        Johannes Berg <johannes@sipsolutions.net>
Cc:     linux-wireless@vger.kernel.org
Subject: Re: [PATCH] mac80211: Simplify queue selection
In-Reply-To: <13154ef7-63ee-cfe7-3ebb-a7f1120985b6@wetzel-home.de>
References: <20220510155828.9406-1-alexander@wetzel-home.de>
 <54589d98-882c-cee4-dd6d-140d50f400d0@nbd.name>
 <13154ef7-63ee-cfe7-3ebb-a7f1120985b6@wetzel-home.de>
Date:   Tue, 10 May 2022 20:26:20 +0200
Message-ID: <87zgjpw8fn.fsf@toke.dk>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

Alexander Wetzel <alexander@wetzel-home.de> writes:

> On 10.05.22 18:10, Felix Fietkau wrote:
>> On 10.05.22 17:58, Alexander Wetzel wrote:
>>> Let iTXQ drivers also register four queues in netdev and move queue
>>> assignment to ndo_select_queue(), like it's done for other drivers.
>>>
>>> This gets rid of a special case in mac80211 and also increases the
>>> chance that when we call skb_get_hash() the skb is still hot in the CPU
>>> buffers.
>>>
>>> Signed-off-by: Alexander Wetzel <alexander@wetzel-home.de>
>> 
>> This has the disadvantage of requiring a redundant sta lookup in the tx 
>> path for iTXQ drivers. I think the CPU cost of that one is probably 
>> higher than any potential gain from calling skb_get_hash a bit earlier.
>
> Found that one, yes. But why do we then not drop ndo_select_queue() for 
> all drivers?
>
> Or maybe just call skb_get_hash() in ndo_select_queue()... But I guess 
> then it would make more sense to move the ndo_select_queue() into 
> netdev, so all drivers get the optimization.

The reason it's an optimisation for mac80211 is that we know
skb_get_hash() will always be called later (when enqueueing into the
TXQ). This is not always the case for other drivers, so that is probably
a dubious optimisation to have in general.

-Toke