Re: [PATCH] Improvev netconsole support for RTL8139 NIC driver
From: Jeff Garzik
Date: Wed Mar 26 2008 - 00:32:44 EST
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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