linux-kernel - Re: [PATCH 3/7] idle, thermal, acpi: Remove home grown idle implementations

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Message-ID: <20131122113300.GM10022@twins.programming.kicks-ass.net>
Date:	Fri, 22 Nov 2013 12:33:00 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Jacob Pan <jacob.jun.pan@...ux.intel.com>,
	Arjan van de Ven <arjan@...ux.intel.com>, lenb@...nel.org,
	rjw@...ysocki.net, Eliezer Tamir <eliezer.tamir@...ux.intel.com>,
	Chris Leech <christopher.leech@...el.com>,
	David Miller <davem@...emloft.net>, rui.zhang@...el.com,
	Mike Galbraith <bitbucket@...ine.de>,
	Ingo Molnar <mingo@...nel.org>, hpa@...or.com,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH 3/7] idle, thermal, acpi: Remove home grown idle
 implementations

On Thu, Nov 21, 2013 at 08:20:36PM -0800, Paul E. McKenney wrote:
> The 6ms to 25ms range should be just fine as far as normal RCU grace
> periods are concerned.  However, it does mean that expedited grace
> periods could be delayed: They normally take a few tens of microseconds,
> but if they were unlucky enough to show up during an idle injection,
> they would be magnified by two to three orders of magnitude, which is
> not pretty.
> 
> Hence my suggestion of hooking into RCU on idle-injection start and end
> so that RCU considers that time period to be idle.  Just like it does
> for user-mode execution on NO_HZ_FULL kernels, so I still don't see this
> approach to be a problem.  I must confess that I still don't understand
> what Arjan doesn't like about it.

Using these patches it would indeed use the RCU idle machinery as per
the normal idle path.

If you can I can add more WARN_ON()s in play_idle() to ensure we're not
called while holding any RCU locks.
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