linux-kernel - Re: [PATCH RFC/RFB] x86_64, i386: interrupt dispatch changes

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Date:	Thu, 6 Nov 2008 10:19:29 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Alexander van Heukelum <heukelum@...tmail.fm>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>,
	Alexander van Heukelum <heukelum@...lshack.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, lguest@...abs.org,
	jeremy@...source.com, Steven Rostedt <srostedt@...hat.com>,
	Mike Travis <travis@....com>,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [PATCH RFC/RFB] x86_64, i386: interrupt dispatch changes


* Alexander van Heukelum <heukelum@...tmail.fm> wrote:

> > | > | Opteron    (cycles): 1024 / 1157 / 3527
> > | > | Xeon E5345 (cycles): 1092 / 1085 / 6622
> > | > | Athlon XP  (cycles): 1028 / 1166 / 5192
> > | > 
> > | > Xeon is defenitely out of luck :-)
> > | 
> > | it's still OK - i.e. no outrageous showstopper overhead anywhere in 
> > | that instruction sequence. The total round-trip overhead is what will 
> > | matter most.
> > | 
> > | 	Ingo
> > | 
> > 
> > Don't get me wrong please, I really like what Alexander have done!
> > But frankly six time slower is a bit scarying me.

the cost is 6 cycles instead of 1 cycles. In a codepath that takes 
thousands of cycles and is often cache-limited.

> Thanks again ;). Now it _is_ six times slower to do this tiny piece 
> of code... But please keep in mind all the activity that follows to 
> save the current data segment registers (the stack segment and code 
> segment are saved automatically), the general purpose registers and 
> to load most of the data segments with kernel-space values. And 
> looking at it now... do_IRQ is also not exactly trivial.
> 
> Also, I kept the information that is saved on the stack exactly the 
> same. If this is not a requirement, "push %cs" is what is left of 
> this expensive (6 cycle!) sequence. Even that could be unnecessary 
> if the stack layout can be changed... But I'ld like to consider that 
> separately.

we really want to keep the stack frame consistent between all the 
context types. We can do things like return-to-userspace-from-irq or 
schedule-from-irq-initiated-event, etc. - so crossing between these 
context frames has to be standard and straightforward.

	Ingo
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