netdev - Re: [PATCH net-next v2 4/4] net: dqs: make struct dql more cache efficient

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Date: Tue, 9 Apr 2024 17:21:49 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Breno Leitao <leitao@...ian.org>
Cc: davem@...emloft.net, pabeni@...hat.com, edumazet@...gle.com,
 linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v2 4/4] net: dqs: make struct dql more cache
 efficient

On Mon,  8 Apr 2024 10:25:56 -0700 Breno Leitao wrote:
> With the previous change, struct dqs->stall_thrs will be in the hot path
> (at queue side), even if DQS is disabled.
> 
> The other fields accessed in this function (last_obj_cnt and num_queued)
> are in the first cache line, let's move this field  (stall_thrs) to the
> very first cache line, since there is a hole there.
> 
> This does not change the structure size, since it moves an short (2
> bytes) to 4-bytes whole in the first cache line.

Doesn't this move the cache line bouncing problem to the other side?
Eric said "copy" I read that as "have two fields with the same value".
I think it's single digit number of alu instructions we'd be saving
here, not super convinced patch 3 is the right trade off...