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

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Message-ID: <47E9D1DC.6030201@garzik.org>
Date:	Wed, 26 Mar 2008 00:32:28 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	David Miller <davem@...emloft.net>
CC:	yang.shi@...driver.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Improvev netconsole support for RTL8139 NIC driver

David Miller wrote:
> First, if you mention CPU instructions executed you're
> totally ignoring what I wrote.  Which is that we're
> about to sit spinning on hundreds of cycles doing a PIO
> read on a status register.
> 
> Those hand full of cycles doing the irqsave/irqrestore don't matter,
> at all.
> 
> Again, you're arguing for 18 cycles or so out of say 800.
> It's peanuts, at best.

No, I hear you.

I'm not focusing on cycles, but list examples of the negative effects of 
doing needless work for the sake of consistency:

* eliminates ability to compile-out spinlocks on UP
* code size increases (even if miniscule)
* CPU instructions in a hot path increases (even if lost in the noise)
* stack usage increases (even if miniscule)

But those are just examples of the principle:  don't do work you don't 
need to do.

I also think spin_lock -> spin_lock_irqsave amounts to a slight loss of 
information, too:  Use of spin_lock() rather than spin_lock_irqsave() 
potentially gives the -rt folks some additional flexibility, by 
advertising a different set of acceptable irq-disablement states.

Is the effect huge in this specific case?  No.

Does that give us license to add needless code to drivers?  No, again, IMO.

	Jeff

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