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

[Jacob Pan <jacob.jun.pan@linux.intel.com>]

On Thu, 21 Nov 2013 12:07:17 -0800
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> 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
> 
currently powerclamp allow idle injection duration between 6 to 25ms.
I guess that is short considering the stall check is in seconds?
	return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;

BTW, by forcing intel_idle to use deepest c-states for idle injection
thread the efficiency problem is gone. I am surprised that cpuidle
would not pick the deepest c-states given powerclamp driver is asking
for 6ms idle time and the wakeup latencies are in the usec.
Anyway, for what i have tested so far powerclamp with this patchset can
work as well as the code before.

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[Jacob Pan]