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Date:	Thu, 3 Apr 2014 17:53:49 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	fweisbec@...il.com, peterz@...radead.org,
	linux-kernel@...r.kernel.org
Subject: Re: Promela/spin model for NO_HZ_FULL_SYSIDLE code

----- Original Message -----
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> To: "Mathieu Desnoyers" <mathieu.desnoyers@...icios.com>
> Cc: fweisbec@...il.com, peterz@...radead.org, linux-kernel@...r.kernel.org
> Sent: Thursday, April 3, 2014 1:41:03 PM
> Subject: Re: Promela/spin model for NO_HZ_FULL_SYSIDLE code
> 
> On Thu, Apr 03, 2014 at 05:33:58PM +0000, Mathieu Desnoyers wrote:
> > ----- Original Message -----
> > > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > > To: "Mathieu Desnoyers" <mathieu.desnoyers@...icios.com>
> > > Cc: fweisbec@...il.com, peterz@...radead.org,
> > > linux-kernel@...r.kernel.org
> > > Sent: Thursday, April 3, 2014 1:21:58 PM
> > > Subject: Re: Promela/spin model for NO_HZ_FULL_SYSIDLE code
> > > 
> > > On Thu, Apr 03, 2014 at 02:57:55AM +0000, Mathieu Desnoyers wrote:
> > > > ----- Original Message -----
> > > > > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > > > > To: "Mathieu Desnoyers" <mathieu.desnoyers@...icios.com>
> > > > > Cc: fweisbec@...il.com, peterz@...radead.org,
> > > > > linux-kernel@...r.kernel.org
> > > > > Sent: Tuesday, April 1, 2014 10:50:44 PM
> > > > > Subject: Re: Promela/spin model for NO_HZ_FULL_SYSIDLE code
> > > > > 
> > > > [...]
> > > > > > Here is the experiment I did on this latest version: I just enabled
> > > > > > the
> > > > > > safety checking (not progress). I commented these lines out
> > > > > > (again):
> > > > > > 
> > > > > >                 /* If needed, wake up the timekeeper. */
> > > > > >                 if
> > > > > >                 //:: oldstate == RCU_SYSIDLE_FULL_NOTED ->
> > > > > >                 //      wakeup_timekeeper = 1;
> > > > > >                 :: else -> skip;
> > > > > >                 fi;
> > > > > > 
> > > > > > compile and run with:
> > > > > > 
> > > > > > spin -a sysidle.spin
> > > > > > gcc -o pan pan.c
> > > > > > ./pan -m1280000 -w22
> > > > > > 
> > > > > > My expectation would be to trigger some kind of assertion that the
> > > > > > timekeeper is not active while the worker is running.
> > > > > > Unfortunately,
> > > > > > nothing triggers.
> > > > > 
> > > > > That is expected behavior.  Failure to wake up the timekeeper is set
> > > > > up as a progress criterion.
> > > > > 
> > > > > > I see 3 possible solutions: we could either add assertions into
> > > > > > other branches of the timekeeper, or add assertions into the worker
> > > > > > thread. Another way to do it would be to express the assertions as
> > > > > > negation of a LTL formula based on state variables.
> > > > > 
> > > > > I did try both a hand-coded "never" clause and LTL formulas without
> > > > > success. You have more experience with them, so perhaps you could
> > > > > make
> > > > > something work.  The need is that if all worker threads go non-idle
> > > > > indefinitely starting from the RCU_SYSIDLE_FULL_NOTED state, the
> > > > > wakeup_timekeeper must eventually be set to 1, and then the
> > > > > timekeeper
> > > > > must start running, for example, the wakeup_timekeeper value must
> > > > > revert
> > > > > to zero.
> > > > > 
> > > > > The problem I had is that the "never" claims seem set up to catch
> > > > > some
> > > > > finite behavior after a possibly-infinite prefix.  In this case, we
> > > > > instead need to catch some infinite behavior after a
> > > > > possibly-infinite
> > > > > prefix.
> > > > > 
> > > > > > Thoughts ?
> > > > > 
> > > > > Me, I eventually switched over to using progress detection.  Which
> > > > > might
> > > > > be a bit unconventional, but it did have the virtue of making the
> > > > > model complain when it should complain.  ;-)
> > > > 
> > > > Here is my attempt at simplifying the model. I use LTL formulas as
> > > > checks
> > > > for the two things I think really matter here: having timekeeping
> > > > eventually
> > > > active when threads are running, and having timekeeping eventually
> > > > inactive
> > > > when threads are idle. Hopefully my Promela is not too rusty:
> > > 
> > > Well, this was the first time I had ever tried using LTL or never claims,
> > > so you are ahead of me either way.  ;-)
> > > 
> > > > 1) When, at any point in the trail, a worker is setup, then we want to
> > > >    be sure that at some point in the future the timer is necessarily
> > > >    running:
> > > > 
> > > > timer_active.ltl:
> > > > [] (am_setup_0 -> (<> timekeeper_is_running))
> > > 
> > > OK, am_setup_0 implies that the timekeeper will eventually be running.
> > > For two threads, this presumably becomes:
> > > 
> > > [] ((am_setup_0 || am_setup_1) -> (<> timekeeper_is_running))
> > 
> > Yes.
> > 
> > > 
> > > > 2) When, at any point in the trail, a worker is not setup, then we
> > > >    want to be sure that at some point in the future, either this
> > > >    thread is setup again, or the timekeeper reaches a not running
> > > >    state:
> > > > 
> > > > timer_inactive.ltl:
> > > > [] (!am_setup_0 -> (<> (!timekeeper_is_running || am_setup_0)))
> > > 
> > > And here the two-thread version should be something like this:
> > > 
> > > [] (!(am_setup_0 || am_setup_1) -> (<> (!timekeeper_is_running ||
> > > am_setup_0
> > > || am_setup_1)))
> > 
> > Yes, I think. Should be double-checked and tested though.
> 
> I did test all four options in the sysidle.sh file with both the original
> and two-thread LTL claims, and got the expected results.  Need something
> to force failures in the other LTL formula, of course.
> 
> > > It would be nice if never claims allowed local variables, as that would
> > > allow looping over the am_setup array.  Or maybe I just haven't figured
> > > out how to tell spin that a given variable should not be considered to
> > > be part of the global state.
> > 
> > AFAIU, we're very limited in what we can put in LTL. You could generate
> > a never claim by hand (see the generated pan.ltl as an example), and maybe
> > have more luck.
> 
> Any time I tried to assign to anything from within a "never" claim,
> it yelled at me.  ;-)
> 
> > > > sysidle.sh:
> > > > #!/bin/sh
> > > > 
> > > > spin -f "!(`cat timer_active.ltl`)" > pan.ltl
> > > > #spin -f "!(`cat timer_inactive.ltl`)" > pan.ltl
> > > > spin -a -X -N  pan.ltl sysidle.spin
> > > > #spin -DINJECT_MISSING_WAKEUP -a -X -N  pan.ltl sysidle.spin
> > > 
> > > I definitely didn't use "-X".  The "spin --help" says "-[XYZ] reserved
> > > for use by xspin interface", so maybe it doesn't matter.  ;-)
> > 
> > Hopefully. I grabbed the "-X" option from the urcu models. I don't
> > remember why we have it there.
> 
> Ah!  ;-)
> 
> > > > gcc -o pan pan.c
> > > > ./pan -f -a -m1280000 -w22
> > > > 
> > > > #view trail with:
> > > > # spin -v -t -N pan.ltl sysidle.spin
> > > 
> > > Interesting.  I have put this into a separate branch.  May I use your
> > > Signed-off-by?
> > 
> > Yes, of course.
> 
> Very good, thank you!
> 
> > > I need to play around a bit more with LTL and progress labels!
> > 
> > For progress, you'll want to add progress labels within the model, and
> > do a LTL formula checking the _np special variable.
> 
> I did use the progress labels, but used -DNP rather than the LTL _np
> special variable.

My general approach is to split the model from the checked LTL claims
as much as possible, so we can use one model for various claims, checking
them one by one. It allows shrinking the max search state space by running
each check within different runs.

For me, doing the progress check within as a separate LTL claim check fits
well in this work flow.

Thanks,

Mathieu

> 
> 							Thanx, Paul
> 
> > Thanks,
> > 
> > Mathieu
> > 
> > 
> > > 
> > > 							Thanx, Paul
> > > 
> > > > sysidle.spin:
> > > > 
> > > > /*
> > > >  * Promela model for CONFIG_NO_HZ_FULL_SYSIDLE=y in the Linux kernel.
> > > >  * This model assumes that the dyntick-idle bit manipulation works
> > > >  based
> > > >  * on long usage, and substitutes a per-thread boolean "am_busy[]"
> > > >  array
> > > >  * for the Linux kernel's dyntick-idle masks.  The focus of this model
> > > >  * is therefore on the state machine itself.  Models timekeeper
> > > >  "running"
> > > >  * state with respect to worker thread idle state.
> > > >  *
> > > >  * This program is free software; you can redistribute it and/or modify
> > > >  * it under the terms of the GNU General Public License as published by
> > > >  * the Free Software Foundation; either version 2 of the License, or
> > > >  * (at your option) any later version.
> > > >  *
> > > >  * This program is distributed in the hope that it will be useful,
> > > >  * but WITHOUT ANY WARRANTY; without even the implied warranty of
> > > >  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> > > >  * GNU General Public License for more details.
> > > >  *
> > > >  * You should have received a copy of the GNU General Public License
> > > >  * along with this program; if not, you can access it online at
> > > >  * http://www.gnu.org/licenses/gpl-2.0.html.
> > > >  *
> > > >  * Copyright IBM Corporation, 2014
> > > >  * Copyright EfficiOS, 2014
> > > >  *
> > > >  * Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > > >  *         Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > > >  */
> > > > 
> > > > // adapt LTL formulas before changing NUM_WORKERS
> > > > //#define NUM_WORKERS 2
> > > > #define NUM_WORKERS 1
> > > > /* Defines used because LTL formula interprets [] */
> > > > #define am_setup_0      am_setup[0]
> > > > #define am_setup_1      am_setup[1]
> > > > 
> > > > byte wakeup_timekeeper = 0; /* Models rcu_kick_nohz_cpu(). */
> > > > 
> > > > #define RCU_SYSIDLE_NOT         0       /* Some CPU is not idle. */
> > > > #define RCU_SYSIDLE_SHORT       1       /* All CPUs idle for brief
> > > > period.
> > > > */
> > > > #define RCU_SYSIDLE_LONG        2       /* All CPUs idle for long
> > > > enough.
> > > > */
> > > > #define RCU_SYSIDLE_FULL        3       /* All CPUs idle, ready for
> > > > sysidle. */
> > > > #define RCU_SYSIDLE_FULL_NOTED  4       /* Actually entered sysidle
> > > > state.
> > > > */
> > > > 
> > > > byte full_sysidle_state = RCU_SYSIDLE_NOT;
> > > > 
> > > > byte am_busy[NUM_WORKERS];  /* Busy is similar to "not dyntick-idle".
> > > > */
> > > > byte am_setup[NUM_WORKERS]; /* Setup means timekeeper knows I am not
> > > > idle.
> > > > */
> > > > 
> > > > byte timekeeper_is_running = 1; /* Timekeeper initially running */
> > > > 
> > > > /*
> > > >  * Non-timekeeping CPU going into and out of dyntick-idle state.
> > > >  */
> > > > proctype worker(byte me)
> > > > {
> > > >         byte oldstate;
> > > > 
> > > >         do
> > > >         :: 1 ->
> > > >                 /* Go idle. */
> > > >                 am_setup[me] = 0;
> > > >                 am_busy[me] = 0;
> > > > 
> > > >                 /* Dyntick-idle in the following loop. */
> > > >                 do
> > > >                 :: 1 -> skip;
> > > >                 :: 1 -> break;
> > > >                 od;
> > > > 
> > > >                 /* Exit idle loop, model getting out of dyntick idle
> > > >                 state.
> > > >                 */
> > > >                 am_busy[me] = 1;
> > > > 
> > > >                 /* Get state out of full-system idle states. */
> > > >                 atomic {
> > > >                         oldstate = full_sysidle_state;
> > > >                         if
> > > >                         :: oldstate > RCU_SYSIDLE_SHORT ->
> > > >                                 full_sysidle_state = RCU_SYSIDLE_NOT;
> > > >                         :: else -> skip;
> > > >                         fi;
> > > >                 }
> > > > 
> > > >                 /* If needed, wake up the timekeeper. */
> > > >                 if
> > > >                 :: oldstate == RCU_SYSIDLE_FULL_NOTED ->
> > > > #ifndef INJECT_MISSING_WAKEUP
> > > >                         wakeup_timekeeper = 1;
> > > > #else
> > > >                         skip;
> > > > #endif
> > > >                 :: else -> skip;
> > > >                 fi;
> > > > 
> > > >                 /* Mark ourselves fully awake and operational. */
> > > >                 am_setup[me] = 1;
> > > > 
> > > >                 /* We are fully awake, so timekeeper must not be
> > > >                 asleep. */
> > > >                 assert(full_sysidle_state < RCU_SYSIDLE_FULL);
> > > > 
> > > >                 /* Running in kernel in the following loop. */
> > > >                 do
> > > >                 :: 1 -> skip;
> > > >                 :: 1 -> break;
> > > >                 od;
> > > >         od
> > > > }
> > > > 
> > > > /*
> > > >  * Are all the workers in dyntick-idle state?
> > > >  */
> > > > #define check_idle() \
> > > >         i = 0; \
> > > >         idle = 1; \
> > > >         do \
> > > >         :: i < NUM_WORKERS -> \
> > > >                 if \
> > > >                 :: am_busy[i] == 1 -> idle = 0; \
> > > >                 :: else -> skip; \
> > > >                 fi; \
> > > >                 i++; \
> > > >         :: i >= NUM_WORKERS -> break; \
> > > >         od
> > > > 
> > > > /*
> > > >  * Timekeeping CPU.
> > > >  */
> > > > proctype timekeeper()
> > > > {
> > > >         byte i;
> > > >         byte idle;
> > > >         byte curstate;
> > > >         byte newstate;
> > > > 
> > > >         do
> > > >         :: 1 ->
> > > >                 /* Capture current state. */
> > > >                 check_idle();
> > > >                 curstate = full_sysidle_state;
> > > >                 newstate = curstate;
> > > > 
> > > >                 /* Manage state... */
> > > >                 if
> > > >                 :: idle == 1 && curstate < RCU_SYSIDLE_FULL_NOTED ->
> > > >                         /* Idle, advance to next state. */
> > > >                         atomic {
> > > >                                 if
> > > >                                 :: full_sysidle_state == curstate ->
> > > >                                         newstate = curstate + 1;
> > > >                                         full_sysidle_state = newstate;
> > > >                                 :: else -> skip;
> > > >                                 fi;
> > > >                         }
> > > >                 :: idle == 0 && full_sysidle_state >= RCU_SYSIDLE_LONG
> > > >                 ->
> > > >                         /* Non-idle and state advanced, revert to base
> > > >                         state. */
> > > >                         full_sysidle_state = RCU_SYSIDLE_NOT;
> > > >                 :: else -> skip;
> > > >                 fi;
> > > > 
> > > >                 /* If in RCU_SYSIDLE_FULL_NOTED, wait to be awakened.
> > > >                 */
> > > >                 if
> > > >                 :: newstate != RCU_SYSIDLE_FULL_NOTED ->
> > > >                         skip;
> > > >                 :: newstate == RCU_SYSIDLE_FULL_NOTED ->
> > > >                         timekeeper_is_running = 0;
> > > >                         do
> > > >                         :: wakeup_timekeeper == 0 ->
> > > >                                 skip; /* Awaiting wake up */
> > > >                         :: else ->
> > > >                                 timekeeper_is_running = 1;
> > > >                                 wakeup_timekeeper = 0;
> > > >                                 break; /* awakened */
> > > >                         od;
> > > >                 fi;
> > > >                 assert(full_sysidle_state <= RCU_SYSIDLE_FULL_NOTED);
> > > >         od;
> > > > }
> > > > 
> > > > init {
> > > >         byte i = 0;
> > > > 
> > > >         do
> > > >         :: i < NUM_WORKERS ->
> > > >                 am_busy[i] = 1;
> > > >                 am_setup[i] = 1;
> > > >                 run worker(i);
> > > >                 i++;
> > > >         :: i >= NUM_WORKERS -> break;
> > > >         od;
> > > >         run timekeeper();
> > > > }
> > > > 
> > > > --
> > > > Mathieu Desnoyers
> > > > EfficiOS Inc.
> > > > http://www.efficios.com
> > > > 
> > > 
> > > 
> > 
> > --
> > Mathieu Desnoyers
> > EfficiOS Inc.
> > http://www.efficios.com
> > 
> 
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
--
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