linux-kernel - Re: Using sched_clock for mmio-trace

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Message-ID: <20070216194059.GF6425@infidigm.net>
Date:	Fri, 16 Feb 2007 14:40:59 -0500
From:	Jeff Muizelaar <jeff@...idigm.net>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Using sched_clock for mmio-trace

On Fri, Feb 16, 2007 at 09:02:44PM +0100, Andi Kleen wrote:
> Jeff Muizelaar <jeff@...idigm.net> writes:
> > 
> > The question is, what api should I be using? I need something that can
> > be called from inside interrupt handlers, and obviously the more
> > accurate and the lower the overhead the better.
> 
> Use do_gettimeofday(). sched_clock() is not for general use
> and only for some very limited use cases and will give you
> unexpected results in several cases.
> 
> There are a few cases where gtod is still a little slow, but these
> are being addressed. In many cases it is fast. In some hardware
> it stays slow, but there is not much that can be done about that
> because of the hardware design.
> 
> It works fine from interrupt handlers and other strange contexts.

Ok, I'll probably use it then. How does the overhead of calling
do_gettimteofday() compare to doing an mmio read/write over PCI express?
i.e. is it going to be a performance problem if I call do_gettimeofday
for every mmio read/write?

Also, is there any good reason blk-trace doesn't use do_gettimeofday()? 

-Jeff
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