linux-kernel - Re: RFC: THE OFFLINE SCHEDULER

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Date:	Fri, 28 Aug 2009 00:27:48 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Rik van Riel <riel@...hat.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Chris Friesen <cfriesen@...tel.com>,
	raz ben yehuda <raziebe@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>, mingo@...e.hu,
	peterz@...radead.org, maximlevitsky@...il.com, efault@....de,
	wiseman@...s.biu.ac.il, linux-kernel@...r.kernel.org,
	linux-rt-users@...r.kernel.org
Subject: Re: RFC: THE OFFLINE SCHEDULER

Gregory Haskins wrote:
> Hi Rik,
> 
> Rik van Riel wrote:
>> Gregory Haskins wrote:
>>
>>> 2) Modify FIFO so that it disables tick by default...update accounting
>>> info at next reschedule event.
>> I like it.  The only thing to watch out for is that
>> events that wake up higher-priority FIFO tasks do
>> not get deferred :)
>>
> 
> Yeah, agreed.  My (potentially half-baked) proposal should work at least
> from a pure scheduling perspective since FIFO technically does not
> reschedule based on a tick, and wakeups/migrations should still work
> bidirectionally with existing scheduler policies.
> 
> However, and to what I believe is your point: its not entirely clear to
> me what impact, if any, there would be w.r.t. any _other_ events that
> may be driven off of the scheduler tick (i.e. events other than
> scheduling policies, like timeslice expiration, etc).  Perhaps someone
> else like Thomas, Ingo, or Peter have some input here.
> 
> I guess the specific question to ask is: Does the scheduler tick code
> have any role other than timeslice policies and updating accounting
> information?  Examples would include timer-expiry, for instance.  I
> would think most of this logic is handled by finer grained components
> like HRT, but I am admittedly ignorant of the actual timer voodoo ;)
> 

Thinking about this idea some more:  I can't see why this isn't just a
trivial variation of the nohz idle code already in mainline.  In both
cases (idle and FIFO tasks) the cpu is "consumed" 100% by some arbitrary
job (spinning/HLT for idle, RT thread for FIFO) while we have the
scheduler tick disabled.  The only real difference is a matter of
power-management (HLT/mwait go to sleep-states, whereas spinning/rt-task
run full tilt).

Therefore the answer may be as simple as bracketing the FIFO task with
tick_nohz_stop_sched_tick() + tick_nohz_restart_sched_tick().  The nohz
code will probably need some minor adjustments so it is not assuming
things about the state being "idle" (e.g. "isidle") for places when it
matters (idle_calls++ stat is one example).

Potential problems:

a) disabling/renabling the tick on a per-RT task schedule() may prove to
be prohibitively expensive.

b) we will need to make sure the rt-bandwidth protection mechanism is
defeated so the task is allowed to consume 100% bandwidth.

Perhaps these states should be in the cpuset/root-domain, and configured
when you create the partition (e.g. "tick=off", "bandwidth=off" makes it
an "offline" set).

Kind Regards,
-Greg

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