linux-kernel - Re: [RFC PATCH V4 2/5] cpuidle: list based cpuidle driver registration and selection

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Date:	Fri, 25 Mar 2011 03:13:04 -0400 (EDT)
From:	Len Brown <lenb@...nel.org>
To:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
Cc:	Trinabh Gupta <trinabh@...ux.vnet.ibm.com>, arjan@...ux.intel.com,
	peterz@...radead.org, suresh.b.siddha@...el.com,
	benh@...nel.crashing.org, venki@...gle.com, ak@...ux.intel.com,
	linux-kernel@...r.kernel.org, sfr@...b.auug.org.au,
	xen-devel@...ts.xensource.com
Subject: Re: [RFC PATCH V4 2/5] cpuidle: list based cpuidle driver registration
 and selection

> > So what do you suggest can be removed?
> 
> Can we use safe_halt() until intel_idle/acpi_idle take over? But what
> if they do not take over?  If safe_halt() is not very bad compared to
> the variants like mwait_idle and c1e_idle, then we can remove the old
> code and no need to move them to default driver.

One reason I'd like a default cpuidle driver is that today
there is a race.  cpuidle registers, but until its driver
registers it will use polling.  go ahead and look:

grep . /sys/devices/system/cpu/cpu*/cpuidle/state0/usage

that should be 0, but it isn't...

> > >Are we suggesting that x86 must always build with cpuidle?
> > >I'm sure that somebody someplace will object to that.
> > 
> > Arjan argued that since almost everyone today runs cpuidle
> > it may be best to include it in the kernel
> > (https://lkml.org/lkml/2010/10/20/243). But yes, we agreed
> > that we would have to make cpuidle lighter incrementally.
> > Making ladder governor optional could be one way for example.
> > >
> > >OTOH, if cpuidle is included, I'd like to see the
> > >non-cpuidle code excluded, since nobody will run it...
> 
> The non-cpuidle code will be the select_idle_routine() and related
> function that cam move to default_driver that register to cpuidle.
> We can load on-demand as module if better routines fail to register.
> Maybe we don't need this at all as discussed in the above point?

Right, though I don't share your fascination with modules.

cheers,
Len Brown, Intel Open Source Technology Center


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