netdev - Re: [PATCH net] net: mvneta: set real interrupt per packet for tx

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Date:	Wed, 6 Jul 2016 07:04:31 +0200
From:	Willy Tarreau <w@....eu>
To:	Marcin Wojtas <mw@...ihalf.com>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	netdev@...r.kernel.org, davem@...emloft.net,
	linux@....linux.org.uk, sebastian.hesselbarth@...il.com,
	andrew@...n.ch, jason@...edaemon.net,
	thomas.petazzoni@...e-electrons.com,
	gregory.clement@...e-electrons.com, nadavh@...vell.com,
	alior@...vell.com, nitroshift@...oo.com, jaz@...ihalf.com,
	Dmitri Epshtein <dima@...vell.com>, stable@...r.kernel.org
Subject: Re: [PATCH net] net: mvneta: set real interrupt per packet for
 tx_done

Hi,

On Wed, Jul 06, 2016 at 04:18:58AM +0200, Marcin Wojtas wrote:
> From: Dmitri Epshtein <dima@...vell.com>
> 
> Commit aebea2ba0f74 ("net: mvneta: fix Tx interrupt delay") intended to
> set coalescing threshold to a value guaranteeing interrupt generation
> per each sent packet, so that buffers can be released with no delay.
> 
> In fact setting threshold to '1' was wrong, because it causes interrupt
> every two packets. According to the documentation a reason behind it is
> following - interrupt occurs once sent buffers counter reaches a value,
> which is higher than one specified in MVNETA_TXQ_SIZE_REG(q). This
> behavior was confirmed during tests. Also when testing the SoC working
> as a NAS device, better performance was observed with int-per-packet,
> as it strongly depends on the fact that all transmitted packets are
> released immediately.
> 
> This commit enables NETA controller work in interrupt per sent packet mode
> by setting coalescing threshold to 0.

We had a discussion about this in January 2015 and I thought I sent a patch
to change it but I can't find it, so I think it was only proposed to some
users for testing. I also remember that on more recent kernels by then
(>=3.13) we observed a slightly better performance with this value set to
zero.

Acked-by: Willy Tarreau <w@....eu>

Willy