linux-kernel - Re: [PATCH] Improvev netconsole support for RTL8139 NIC driver

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Date:	Tue, 25 Mar 2008 20:30:43 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	jgarzik@...ox.com
Cc:	yang.shi@...driver.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Improvev netconsole support for RTL8139 NIC driver

From: Jeff Garzik <jgarzik@...ox.com>
Date: Tue, 25 Mar 2008 23:14:03 -0400

> David Miller wrote:
> > From: Jeff Garzik <jgarzik@...ox.com>
> > Date: Tue, 25 Mar 2008 22:23:24 -0400
> > 
> >> This is bogus -- you should never need to slow down the hot path in such 
> >> a way.
> > 
> > Slow down in what way?  Even on x86 saving the flags is just
> > about as expensive as a plain sti/cli.
> 
> Replacing spin_lock() [current 8139too.c] with spin_lock_irqsave() 
> results in a larger interrupt handler... more CPU instructions for the 
> same result.

Jeff, please be realistic.

These interrupt handlers about to do a PIO on a status register, which
will consume on the order of a few hundred cpu cycles.

Counting an I-cache line or two, or 18 cycles here or there,
is immaterial by comparison.
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