linux-kernel - Re: [PATCH net] net: stmmac: Fix NAPI poll in TX path when in multi-queue

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Message-Id: <20190214.090107.1088098621242009425.davem@davemloft.net>
Date:   Thu, 14 Feb 2019 09:01:07 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     jose.abreu@...opsys.com
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        joao.pinto@...opsys.com, peppe.cavallaro@...com,
        alexandre.torgue@...com
Subject: Re: [PATCH net] net: stmmac: Fix NAPI poll in TX path when in
 multi-queue

From: Jose Abreu <jose.abreu@...opsys.com>
Date: Wed, 13 Feb 2019 18:00:43 +0100

> Commit 8fce33317023 introduced the concept of NAPI per-channel and
> independent cleaning of TX path.
> 
> This is currently breaking performance in some cases. The scenario
> happens when all packets are being received in Queue 0 but the TX is
> performed in Queue != 0.
> 
> I didn't look very deep but it seems that NAPI for Queue 0 will clean
> the RX path but as TX is in different NAPI, this last one is called at a
> slower rate which kills performance in TX. I suspect this is due to TX
> cleaning takes much longer than RX and because NAPI will get canceled
> once we return with 0 budget consumed (e.g. when TX is still not done it
> will return 0 budget).
> 
> Fix this by looking at all TX channels in NAPI poll function.
> 
> Signed-off-by: Jose Abreu <joabreu@...opsys.com>
> Fixes: 8fce33317023 ("net: stmmac: Rework coalesce timer and fix multi-queue races")

No this isn't right.

The TX interrupt events for Queue != 0 should clean up the TX packets
on those queues.

Furthermore you are breaking the locality of the TX processing.

I'm not applying this, sorry.