linux-kernel - Re: [RFC PATCH] x86-64: software IRQ masking and handling

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Message-ID: <20100711192615.GB21598@elte.hu>
Date:	Sun, 11 Jul 2010 21:26:15 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Tejun Heo <tj@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RFC PATCH] x86-64: software IRQ masking and handling


* Tejun Heo <tj@...nel.org> wrote:

> Hello,
> 
> This is something suggested by Rusty Russell a while ago.  It makes IRQ 
> masking a software switch like preemption or softirq enable/disable.  
> Hardware interrupt masking (cli/sti) and delivery are decoupled from actual 
> IRQ handling.  IRQ disabling is done by single instruction moving 1 to a 
> percpu variable.  Enabling is similar but it should check whether there's 
> any pending interrupt to handle.
> 
> This change greatly reduces the number of hardware IRQ masking 
> manipulations.  cli/sti still being somewhat costly operations (I hear 
> nehalem is better tho), this should be able to improve overall performance, 
> especially on paravirts.

Not just Nehalem but on various AMD CPUs it was in the below-10-cycles range 
for years.

Note that we tried this in -rt, but the pain and trouble (and, often, code 
bloat) was not worth the trouble. The PUSHF/POPF/CLI/STI instructions are 
really simple and short in the instruction stream - without disturbing other 
registers.

	Ingo
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