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

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Date:	Thu, 21 Nov 2013 12:07:17 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Jacob Pan <jacob.jun.pan@...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 11:45:20AM -0800, Arjan van de Ven wrote:
> On 11/21/2013 11:19 AM, Paul E. McKenney wrote:
> >On Thu, Nov 21, 2013 at 08:21:03AM -0800, Arjan van de Ven wrote:
> >>On 11/21/2013 8:07 AM, Paul E. McKenney wrote:
> >>>As long as RCU has some reliable way to identify an idle task, I am
> >>>good.  But I have to ask -- why can't idle injection coordinate with
> >>>the existing idle tasks rather than temporarily making alternative
> >>>idle tasks?
> >>
> >>it's not a real idle. that's the whole problem of the situation.
> >>to the rest of the OS, this is being BUSY (busy saving power using
> >>a CPU instruction, but it might as well have been an mdelay() operation)
> >>and it's also what end users expect; they want to be able to see
> >>where there performance (read: cpu time in "top") is going.
> >
> >My concern is keeping RCU's books straight.  Suppose that there is a need
> >to call for idle in the middle of a preemptible RCU read-side critical
> >section.  Now, if that call for idle involves a context switch, all is
> >well -- RCU will see the task as still being in its RCU read-side critical
> >section, which means that it is OK for RCU to see the CPU as idle.
> >
> >However, if there is no context switch and RCU sees the CPU as idle,
> >preemptible RCU could prematurely end the grace period.  If there is no
> >context switch and RCU sees the CPU as non-idle for too long, we start
> >getting RCU CPU stall warning splats.
> >
> >Another approach would be to only inject idle when the CPU is not
> >doing anything that could possibly be in an RCU read-side critical
> >section.  But things might get a bit hot in case of an overly
> >long RCU read-side critical section.
> >
> >One approach that might work would be to hook into RCU's context-switch
> >code going in and coming out, then telling RCU that the CPU is idle,
> >even though top and friends see it as non-idle.  This last is in fact
> >similar to how RCU handles userspace execution for NO_HZ_FULL.
> >
> 
> so powerclamp and such are not "idle".
> They are "busy" from everything except the lowest level of the CPU hardware.
> once you start thinking of them as idle, all hell breaks lose in terms of implications
> (including sysadmin visibility etc).... (hence some of the explosions in this thread
> as well).
> 
> but it's not "idle".
> 
> it's "put the cpu in a low power state for a specified amount of time". sure it uses the same
> instruction to do so that the idle loop uses.
> 
> (now to make it messy, the current driver does a bunch of things similar to the idle loop
> which is a mess and fair to be complained about)

Then from an RCU viewpoint, they need to be short in duration.  Otherwise
you risk getting CPU stall-warning explosions from RCU.  ;-)

							Thanx, Paul

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