linux-kernel - Re: [PATCH] Extend mwait idle to optimize away CAL and RES interrupts to an idle CPU -v1

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Message-ID: <1330332354.11248.32.camel@twins>
Date:	Mon, 27 Feb 2012 09:45:54 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Venki Pallipadi <venki@...gle.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Aaron Durbin <adurbin@...gle.com>,
	Paul Turner <pjt@...gle.com>,
	Yong Zhang <yong.zhang0@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Extend mwait idle to optimize away CAL and RES
 interrupts to an idle CPU -v1

On Thu, 2012-02-23 at 11:34 -0800, Venki Pallipadi wrote:

> > > +             local_bh_disable();
> > > +             local_irq_disable();
> >
> > That local_bh_disable() is completely superfluous, disabling IRQs
> > already kills bh.
> 
> The reason for local_bh_disable/enable was to check for potential
> softirqs after these interrupt.

Why is that needed? We never wrap local_irq_disable() in bh muck?


> > Why doesn't ipiless_idle_exit() call this? That would keep it isolated
> > to just those idle routines that actually use mwait instead of bothering
> > the generic idle paths with this.
> 
> ipiless_idle_exit is called in the inner most part of idle entry exit.
> In mwait case we may not even have interrupts enabled at that time and
> there is code that does idle residency timing etc which will get
> impacted if we start doing the pending ipi work there. Doing it at
> higher level along with things like enter-exit tickless felt nicer.

But but but, an actual interrupt can be handled before the instruction
after mwait, so why would this be a problem?

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