linux-kernel - Re: [PATCH] make poll_idle behave more like the other idle methods

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Message-ID: <alpine.LFD.1.10.0808281215190.3243@apollo.tec.linutronix.de>
Date:	Thu, 28 Aug 2008 12:40:03 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Joe Korty <joe.korty@...r.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] make poll_idle behave more like the other idle methods

On Thu, 28 Aug 2008, Ingo Molnar wrote:
> * Joe Korty <joe.korty@...r.com> wrote:
> 
> > Make poll_idle() behave more like the other idle methods.
> > 
> > Currently, poll_idle() returns immediately.  The other
> > idle methods all wait indefinately for some condition
> > to come true before returning.  poll_idle should emulate
> > these other methods and also wait for a return condition,
> > in this case, for need_resched() to become 'true'.
> > 
> > Without this delay the idle loop spends all of its time
> > in the outer loop that calls poll_idle.  This outer loop,
> > these days, does real work, some of it under rcu locks.
> > That work should only be done when idle is entered and
> > when idle exits, not continuously while idle is spinning.
> 
> i'm wondering, what's the motivation, have you actually seen
> anything bad/undesired happen due to that?

Hmm, two observations:

1) I think Joe is right that idle_poll should behave like any other
idle function.

2) I wonder whether the work you observe is something we should
investigate. The only code which does work in the idle loop is
rcu_check_callbacks(). What kind of work load scenario do you have
which makes the rcu_check_callbacks() called ?

Thanks,

	tglx
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